I don't recommend having a genset with a different fuel source than the your Class A, B or C as you will be refueling the propane WAY more often than you like. Having only 1 large fuel tank is easier and lasts much longer than the 2 separate sources. The genset is also rigged so that if your fuel tank drops below 1/4 tank the genset will quit. This prevents you from running out of fuel and being stuck out in the desert.
My 1st Class A was diesel and the genset propane. It has a large 100# propane tank and we still ended up driving the coach to a propane dealer way too often. We were camping 1 very hot weekend and had to run the generator the entire weekend so we burned through 100# in 48 hours.