I can sure see why a manufacturer would rather install a propane generator over a gas generator. No fuel tank to build, certify and do whatever else the fed gov requires.
I know when I had motorhomes I ran across a few diesel motorhomes with propane generators installed by some manufacturers because diesel generators were so much more expensive. However, folks that had them usually didn't much care for them because they used so much propane and it's more difficult to get the propane tank on a motorhome filled. Diesel generators just draw from the existing 100 gallon diesel tank.
Now I've heard of a trailer with a propane generator but I really don't feel all that lucky today. :B