I have a 07 Melbourne 29D and have full-time for 6 years in it. If you mount a second unit it would be too close to the exhaust unit, you could replace the unit with a larger unit cost about $750. Part of the cost is it has to be done by a certified Tech.
We last one month on propane with a small electric heater 1200watt, summer two to three months we out side BBQ. The Melbourne comes with a quick disconnect for BBQ use it with a small 20lb bottle with a regulator. Do not transport Propane bottle in the Motor Home.
Thank have fun traveling
Bobby J