on their fifth wheel or in their fifth wheel? i would expect the vast majority of generators permanently installed in a fifth wheel to be propane powered, with the exception of toy haulers. the fifth wheels have propane tanks installed that can power the generator. while the toy haulers also have propane they may also have a gasoline tank that the generator could use for fuel. if you are talking about taking an external generator and somehow installing it on the fifth wheel, perhaps the bumper, i would expect them to be gasoline using the tank installed on the generator itself.
2015 cardinal model 3825fl
2015 dodge ram 3500 dually