You would have to run another 12V DC power source to furnace fan motor and switch the existing power source off because furnace fan only runs when t-stat calls for heat...which also causes the gas valve to open, spark electrode to light main flame etc.
And your electric heater would not put out enough HOT air to sufficiently heat the underbelly, tanks etc.
Are you going to be stationary or traveling?
Stationary......have a local propane company set up 100$ propane tank
You need the propane furnace as primary and electric heaters as secondary....unless you decide to install the electric heater modification to your existing furnace discharge plenum.
Heat source has to be at plenum in order to blow HOT air thru out rig.