PHOENIX, Arizona. Driving here on Interstate 10, the highway that crosses south Texas from San Antonio to El Paso, I see a steady stream of RVs running east. My bet is that they are leaving Arizona earlier than usual because it's unseasonably hot. The radio is reporting temperatures nearing 100 and it's still March. I won't get a chance to visit the big RV parks in Harlingen on this trip--- known for its birders--- but I know the RVers in Kerrville and all the towns running West of San Antonio are still comfortable.

I'm driving into the heat because of a word. Sunburst.

If you visit RV resorts in Florida you'll see it. There is a chain of them: 25 in Florida, 6 in Texas, 3 in California, and 1 in Arizona. They are listed on the web at www.rvonthego.com, complete with a reservation system and special offers, a network put together by David Napp and his partners.

Mr. Napp lives and works in Phoenix.

He is 43, relaxed, and a recovering attorney. He's been investing in RV parks for nearly 10 years. I asked how he got started.

"We purchased our first properties in 1996, three RV parks and three mobile home properties. One of the RV parks was in Port Charlotte, Florida; two were in the Rio Grande Valley, Texas.

"We knew that these properties could generate good cash flow. The fundamental thing about an RV resort is that you're accommodating affordable vacations, NOT affordable housing," he said. The distinction was important, he said, because there were fewer problems and more credit worthy customers.

"It's discretionary." That, he pointed out, made the RV park a choice, not a necessity. "And the core fundamental is the Park Model. (A park model is a small manufactured home that is classified as an RV but seldom, if ever, moves.) Did you know that 60 percent of the owners pay cash? Many also own an RV, or they may trade an RV in to buy the Park Model. Often the value of the RV they trade in will exceed the cost of the Park Model.

"So, now you've got a debt free Park Model on leased land," he explained, "and the question is what makes them keep coming back?

"Well, there are three things in life that are certain: death, taxes, and snow. They'll come down in the winter, to avoid the snow."

"The first investor question we usually hear is, 'What's the effect of rising gasoline prices?"

"Answer: on a Park Model, nothing." Someone who owns a Park Model, he points out, isn't driving a truck that gets 5 miles to the gallon. "So there is great stability to the Park Model person. They will always be wanting to get away from winter."

I asked if he had developed any new parks.

"We've bought all existing parks and we've added sites within those parks. We've found that's the way you have to go. We couldn't buy a piece of land and develop it," he said. He explained that it was often difficult to get zoning for new parks. He also noted that it was getting more difficult because some parks were being bought out for "higher use."

What did he see as the future?

"Our core business will always be the Park Model. The customer always starts as an RV owner because freedom to travel is a compelling idea. That's what gets people into it.

"As they travel they find places they like more than any others. Then they want to spend two months in Arizona, or Florida, or Texas. They often find another couple they like and they stay."

I asked how he found RVers for his parks.

"We make a lot of effort to serve the customer because 70 percent of our business is based on repeat customers and referrals. We try to provide the experience the customer is looking for. Of course, there are always sources like Woodall's and Trailer Life, as well.

"We have booths at RV shows, our website, and we do e-mail blasts. We spend a lot of our marketing dollars to capture that 30 percent."

I asked how he saw the future.

"You can buy a Park Model for less than $50,000 and your land rent will be about $3,000 (a year). I think that's one of the better choices a person can make. It's a trend that's going to keep on going.

We've seen multiple Park Model owners," he said.

Indeed, while a long term ownership/rental commitment doesn't make sense for a person with two weeks of vacation, it may be the affordable way to spend three or four months a year vacationing as a retiree or semi-retiree. Do the math and you learn that a total investment of $80,000--- less than half the resale price of a conventional house--- will buy you two new high quality Park Models with total annual out-of-pocket expenses of about $6,000 a year for two land rents, one North and one South.

Recently, Mr. Napp pointed out, he had partnered with Mobile Home Communities (ticker MHC), a manufactured housing REIT run by Sam Zell. Together, they had gone from 28 properties to 68, with northern and southern locations. "We're putting something together that will be representative of what people want as they get older.

"Today's baby boomers want more flexibility, more activities. What we've done in partnering with MHC is allow ourselves to create products that meet needs. We'll be able to create flex plans--- trading models from one park to another. And that means we'll be able to add new customers.

One the web:

Woodall's Trailerlife

Sunburst and Encore RV Parks

MHC stock information at MSN Money

MHC website