Hi GoPackGo,
Replace the return air grill on the furnace with a fan. On my rv that circulates warm "living space" air under the cabinets and pushes some air along the furnace duct work. I chose to use a dual window fan unit which is controlled by a mechanical thermostat that lives beside the water pump next to the outside wall of the RV.
The fan unit draws just 23 watts and works better than a 1500 watt fan based heater which I tried using inside the cabinet.
GoPackGo wrote:
2. If it's really cold you will want to make sure you get some heat to the underbelly area to keep your lines and tanks from freezing.
If you primarily use elec heaters then you may want to somehow circulate the warmer cabin heat to those unheated areas. The problem you can run into is that if the elec heaters are keeping the cabin temp nice and warm, then the furnace doesn't kick on which means no heat is getting to your water tanks/lines.