[folivier]

It wouldn't hurt to run the engine preheater for an hour before you start it. The preheater is just a circulation loop + 12VDC pump that runs through the engine coolant to warm up the engine.
While you're driving the coach does the tap water get hot? If so then the aqua hot is circulating through the engine coolant loop and the heaters should work while driving the coach. The AH heaters should run on 12VDC.