NEW YORK (CNN/Money) - Clark Griswold could not resist the lure of traveling across the country via the "family truckster" in "National Lampoon's Vacation" -- but he is not alone.

Americans are flocking to recreational vehicles in a post-Sept. 11 travel environment, sending sales of RV makers soaring -- and their stocks have followed.

Shares of Thor Industries Inc. leads the pack -- now more than triple its lows of last September, while rival RV makers Winnebago Industries Inc. and Monaco Coach Corp. have more than doubled. Fleetwood Enterprises remains the laggard in the sector -- up just 8 percent from its post-Sept. 11 low.

Although a reluctance to board planes and wait in airports may be seen as the reason behind the rise in RV stocks, Sept. 11 only served as a catalyst, boosting a recovery already in progress, said Scott Stember, an analyst with New York-based Sidoti & Co., who has "buy" ratings on Monaco, Winnebago and Thor.

"There were signs of a recovery [in the sector] before September," Stember added. "These are planned purchases, and we're seeing a release in pent up demand, especially among baby boomers."

Sidoti & Co. does not have an investment banking relationship with Monaco, Thor or Winnebago, nor does Stember own any of the stocks.

A Winnebago RV. Analysts agree Winnebago shares have the most growth potential.

Indeed, sales of recreational vehicles jumped 16 percent in April from a year earlier, in line with average increases this year, after an increase of only 2 percent in 2001, according to the Recreational Vehicle Industry Association.

Most of the RV buying is being done by baby boomers, who are reaching their peak buying years of 50 or older, and another 10,000 Americans turn 50 every day, said Ken Sommer, spokesman for the RVIA, citing the latest U.S. census figures.

Another factor adding to the jump in sales is lower interest rates. "Not only do lower rates make it easier to finance these purchases, but they allow dealers to get more inventory onto their lots," said Stember.

With price tags ranging from $3,500 to $1 million, many buyers take out loans to purchase RVs and most of these loans can be used as second home tax deductions.

RV stocks have reflected the upswing in the industry, but analysts agree that most stocks still have growth potential.

"Before 9/11 these stocks were trading at unprecedented low valuations," Stember continued. "They're now in the growth phase of a new cycle. The stocks will probably not double from here, but there's room for growth of another 25-to-30 percent."

An indication of Thor Industries' recent rally is its upcoming stock split. The company, which is the largest maker of fifth-wheel units, or RVs that are towed behind a truck, said Tuesday it will split its stock 2-for-1 to shareholders of record as of Wednesday.

In addition to its split, the Jackson Center, Ohio-based company said preliminary May sales, including its recently acquired Keystone unit, soared 72 percent to $138.5 million, compared to a year earlier. Excluding the Keystone unit, Thor said sales climbed 22 percent from a year ago.

"We expect their sales this year to exceed those from the last time the industry peaked in late 1999," said Rikard Ekstrand, portfolio manager with the FPA Capital fund, which has a large stake in Thor and Fleetwood. "Thor (THO: up $1.86 to $72.51, Research, Estimates) is reasonably valued, but it's not the bargain it was in the high $20s."

"These stocks historically have price-to-earnings ratios in the high teens to low 20s, an I expect they'll return to that level," Stember added.

Shares of Thor trade at 18.7 times 2003 earnings estimates of $3.74 a share, according to First Call, putting them at the low end of Stember's valuation range.

Although Thor has been the best performer in the sector, analysts agree shares of Winnebago Industries, whose name has become synonymous with recreational vehicles, has the most to gain at this point.

"It's the largest producer of gas-powered RVs, which was the hardest-hit segment during the downturn in the industry, and it will recover the most as well," Stember continued. "It has the most wind at its back right now."

Bob Straus, member of the Icon Consumer Discretionary fund portfolio management team, which holds Winnebago shares, said, "Winnebago appears to be the best positioned in the sector. It has the strongest management, which has consistently delivered results year over year."

The Forest City, Iowa-based company said Wednesday its third-quarter profit jumped 50 percent to 90 cents a share -- trouncing Wall Street estimates of 75 cents, according to research firm First Call. Winnebago said third-quarter sales climbed 26 percent to $246.6 million.

"Excellent acceptance of Winnebago Industries' motor homes, as evidenced by our continued market share gains, as well as improvements in consumer confidence levels and sustained low interest rates have all contributed to the success of our third quarter," said the company's chairman, CEO and president, Bruce Hertzke, in a prepared statement.

Winnebago shares are trading at 15.6 times 2003 estimate earnings of $2.98 -- below the historical range of the high teens-to-low 20s.

"The group as a whole is undervalued by 10-to-15 percent," added Icon's Straus. "But we think Winnebago (WGO: down $0.56 to $45.24, Research, Estimates) is undervalued by about 20 percent."

It may not be the French Riviera, but Monaco Coach's RVs also sport a touch of class. The Coburg, Ore.-based company focuses on diesel engine vehicles, which last longer and tend to be more expensive than gas-powered RVs, according to Bob Male, portfolio manager with the Buffalo Small Cap fund, which holds Monaco shares.

"The engines last longer and first-time buyers will often start with a gas engine, then trade up to a more expensive diesel," said Male. "By the time buyers get around to their second RV, they're often about 60 years old -- and this demographic has the most earnings power."

Monaco's high-end focus led second-quarter profit to soar 83 percent to 33 cents a share in the quarter ended March 30.

The company's stock has followed the rise in income, but Male said it's not at the point where it is too expensive. "We think their earnings estimates are conservative, so its forward-looking P/E of about 12.5 may be a little misleading. It's probably trading around 11 times 2003 earnings."

"Monaco (MNC: down $0.82 to $22.84, Research, Estimates) shares looked a little expensive around $30," said Jeff Tryka, analyst with Ranier Research, who has a "hold" on Monaco. "But they've seen a correction lately and we think they're more attractive in the low $20s."

While stocks of other RVs makers have headed toward higher ground, shares of Fleetwood Enterprises have been stuck in a rut.

"The company paid a lot to make some acquisitions in the [mobile] home industry and the company eventually used convertible stock to pay for the acquisitions and the dilution limited the upside potential of the stock," said FPA's Ekstrand. "But we do think the stock is certainly attractive."

Slack demand in the mobile home business has also hurt Fleetwood shares. "Lenders have backed away from the market and there's just no activity," said William Gibson, analyst with Banc of America Securities, who has a "market performer" rating on Fleetwood.

In early May, the Riverside, Calif.-based company said it expects to report a loss in the fourth quarter and in fiscal year 2002, due to "difficulties in the manufactured housing environment and continued losses in our retail and travel trailer divisions."

Although its struggling in the housing market, Fleetwood's RV operations remain robust. The company said its preliminary fourth-quarter sales increased by 28 percent from last year to $369 million, with motor homes leading the way with a 46 percent increase from a year earlier.

Despite the troubles in the mobile home business, FPA's Ekstrand says the problems may have made Fleetwood shares a bargain. "The stock looks cheap down here," he said.