LPG conversions were fairly common before the days of port fuel injection. They were easiest were in the carburetor era, replacing the carb with a LPG throttle body injection piece.
There are firms today doing dual-fuel conversions, either CNG or LPG with gasoline on modern fuel injection vehicles (mostly medium duty trucks, which fall outside a lot of EPA restrictions that apply to lighter vehicles). Cost of a dual fuel conversions is typically in the $6000 to $15,000 range (it is a whole lot less as a factory option).
Your gas motorhome is built on a bare medium duty truck chassis. It is a matter for finding a supplier for the conversion and working out a price.