You probably won't find a diesel generator in a Class C, especially on those smaller chassis. Diesels tend to be larger and heavier than equivalent gas or propane engines, and both space and weight are at a premium.
Diesel fuel contains about 30% more BTUs per gallon than propane, so everything else being equal, a diesel generator will use about 30% less fuel by volume than a propane generator.
Of greater concern is the size of your propane tank vs. the diesel tank, and where you will get the propane tank refilled when it's empty. Keep in mind your stove, refrigerator, water heater and furnace will stop working if you drain the propane tank.
On the plus side, propane is virtually odorless when it's burned in an engine You can't say the same for diesel.