A couple of clarifications.
First, You mention not wanting to run a dedicated power line for your new floor heating pads. I'm assuming you mean 'not running 12V decidated power liner for your new floor heating pad THERMOSTAT' - you'll still need to run new 120V lines for the actual power pads.
Second, you don't mention what year your Bounder is but I believe the older ones only had 30A electrical. If you only have 30A electrical, you might not be able to run the heat pads without sharing the circuit with something else (e.g. AC unit (which I guess would probably work unless your AC units also have heat pads)).
Third, you mention wanting to disable the LPG furnace. For resale (and even for your own use), I'd recommend not doing that. Radiant heat is nice but are you going to cover the whole motor home (e.g. bedroom, bathroom, slide floors, etc) - you might find that while the under floor heat is nice where the pads are, you still need supplemental heat in other places. The other thing to consider is that in general, mhs are not insulated like stick houses - in-floor heating might not work very well in a motor home because you lose too much heat.