Most class "C"'s are built on a cutaway Ford or the Chevrolet chassis, very few are Diesel, the majority are gasoline and the generators are also powered with gasoline, having a separate propane tank is a waste of space and weight, specially today with slide outs that eat into the carrying capacity of the units.
Diesel "C"'s tend to be very loud in the cabin since part of the engine and all of the transmission are in a very thin dog house next to the occupants, I would not like the noise and racket next to me for several hours.
The generator taps into the chassis fuel tank and is designed to stop delivering fuel to the generator when the fuel tank reaches 1/4 full, this is done to prevent the tank being completely emptied by running the generator.
If you get one of the big "C" units built on a cargo carrying chassis then you have the option of having more fuel tanks since the chassis will handle more weight.
navegator