an Onan 5500 like the factory one installed in our camper list for around $5,000 but with tax/shipping can be bought for around $4,000 .... to that you'd have to add hardware for exhaust along with modifying the compartment it goes in, sound proofing, remote start switch and fuel monitor panel (if desired) .... an automatic power switch would run in the $250 neighborhood .... buying and mounting a fuel tank or running a propane would have to be added along with the labor which at RV dealerships usually runs upward of $100 an hour .... in short, even if you can do all of the labor it will be an expensive project
Jim
'07 Freightliner Sportchassis
'06 SunnyBrook 34BWKS