Our family business used to be propane until the folks sold to a national company back in the 90's. Literally the first thing I drove when I was a little kid was a Wheel Horse with a bbq tank on the back it ran on. Used it on everything from cars, pickups, medium duty trucks and race cars. So I have the experience, no problem there.
Propane is a very simple fuel to use, nothing special to do once the initial conversion is done. It burns cleaner than gasoline, emissions are practically zero compared to a gas or diesel engine, which makes sense in today's "green" craze. I can guarantee Schwan's is getting a boatload of government subsidies or tax credits for every new "alternative fuel" vehicle they put on the road. They gotta be loving Obama. The fuel is very clean, as in no contaminants in the fuel, nothing to gum up the engine, and we used to experience double the engine life of a comparable gasoline engine as a rule of thumb. I remember pulling the heads off of a 366 big block chevy in one of our C-60 medium duty delivery trucks (needed a valve job) that had 375,000 miles on it. The engine literally looked like new inside, no gunk or carbon buildup, we did a valve job put the heads back on and ran it another 100,000 miles til the chassis wore out. Those are around town delivery miles, not over-the-road, and mind you this is a gas motor, not a diesel.
Mileage is comparable to the same vehicle on gasoline, properly tuned of course. In my case my Chevy 3500 dually gets about 11 mpg empty, and 6.5 mpg towing my 20,000# gooseneck on gasoline. The 6500 got over 10 mpg empty driving it back from Wisconsin where we bought it to Ohio, so I expect the towing mileage will be about the same as my dually as well, minus something for the weight of the box I want to build.
Power again is comparable to the same engine on gas. In the old days propane had a reputation for lower power, but you just need to tune a little differently than gas and most people don't know how. For example in my race motors I would run 45-48 degrees total timing depending on how hard I wanted to lean on it, compared to maybe 38 degrees on a gasoline race motor. Different heat range in the plugs, etc. In those days it would have been a propane carb, but these new Schwans trucks run direct port liquid propane injection through the stock injectors and stock computer program, they just change out the fuel rails, lines, (and tank of course) as they are running it at about 160 psi to the injector. Other aftermarket conversion kits for pickups and cars can use a stand alone computer or wire into the stock computer, and can actually run dual-fuel where you can switch over to gasoline at the flip of a switch if needed.
My truck has dual tanks and can haul 160 gallons, so I should have almost 1000 mile range worse case, and every flying J has propane. Some of the Schwans trucks only have a single tank at around 100 gallons, which still gives a good range.
There are plenty of Schwan's trucks out there as bare cab/chassis if you look. They put the refrigerator body on a new chassis at around 175,000 miles from what I have seen, which leaves plenty of life for what we use them for, and engine life is good as previously noted. They are generally cheap because the propane scares off a lot of folks. The dealer I bought mine from converts most of them back to gas before he sells them, which just involves fuel rails, lines, tank, and fuel pump. Most of them are 4500 and 5500 chassis, which are great for very small toters, or a western hauler type deal, but there are some 6500's but they are a little harder to find.
I paid $5000 for mine with 180,000 on the clock, and it is in pristine condition. But the Schwan's gold paint has got to go! I am pleased with mine for the price, and you can get the 4500/5500 trucks cheaper than that. Try finding a dually pickup in good shape for $5000.
Another added bonus for an RV conversion, is you don't need another propane tank, you can run the coach off the same tank, and you can get a propane powered RV generator far cheaper than a diesel model, and again run off of the same tank.