In my opinion it would be whichever mates up best with the chassis. If the chassis engine runs on diesel, I would go with a diesel generator. If gas, a gas generator.
I don't have any experience with LP generators except for the 15kw emergency generator here at home station. I do know it runs cleaner with less oil and sparkplug fouling, and propane doesn't evaporate or gum up like gas or diesel. But a propane generator for my MH would take more propane than I can carry without seriously upgrading my propane tank. Plus propane fired generators have lower output than a same sized gas or diesel generator.
Good luck with your decision, and post back to let us know how it goes.
I would go with either Diesel(preferred) or gas and the last choice propane. If your into boondocking you will be running for propane much sooner than with either of the other two. Plus most of the propane in a C is built in so the C has to move to the propane with gas or diesel a refill can can be used.