I have the Onan 5500 that was factory installed with a 10 gallon gasoline tank built in also. I don't think RV manufacturers offer the gasoline option now but not sure. I do know my gas 5500 works great, runs both a/c's and everything else I use at the same time and runs a bit north of 20 hours on 10 gallons depending load. 11 years full time use, 650 hrs on the Onan and not looking back.
I do use non-ethanol gas and still put stabilizer in every tank plus running the generator every month for 1 hour under load.