Ah, OK, that helps
. If you have a 5th-wheel then you already have a propane supply so that's an obvious advantage, and unless you have a very large 5th-wheel then you're probably going to use one of the Onan Microquiet line so noise level will be about the same regardless of generator fuel type. Fuel usage will be slightly less in a diesel but as above you'll probably never come close to making the initial purchase difference back in fuel savings, especially since propane tends to cost a bit less than diesel which nets much of the savings out.
If you don't have to have a built-in unit then you might want to consider one of the Honda/Yamaha inverter generators. They are about as quiet and fuel efficient as you are going to get and will cost less than half the price of a built-in model.
Also, if you are going to boondock much make sure you get a quality, multistage battery charger/converter. That can matter even more than the generator choice since a good charger can easily mean the difference between running the generator 1-2 hours vs. all day in order to bring the batteries back up.