LK,
The Bighorn has the same thing, but the basement is really not heated at all.
Heated underbelly is a stretch and evidenced by many reports of freezing pipes. Here are the problems:
1. The heat vent that goes to the underbelly is a small 1 - 1 1/2" pipe.
2. The vent does not have much force behind it and absolutely has no way of heating the entire underbelly or even the front half.
3. The vent only drops down under the black tank and into the dropped frame area and not into the rear area, which is prone to freeze and no way the heat from the weak vent can get to. Furthermore, why dump heat on the lack and grey tanks only? It should be dumped where the fresh water is.
4. Hear rises, so the heat that is put into the front belly area just rises back up into the area where the heater is located.
5. The fresh water line is uninsulated. It lies on the bottom of the underbelly with only a small foil insulator protecting it.
Solutions for current owners:
1. Extend the vent to get heat on the waste valves and fresh water area or add a second heater to the rear area. I added a Golden rod between the fresh water tanks and the galley tank. The Rod is good for 500 sq feet and is much safer then a light bulb or ceramic heater. I'll let you know how it works in our upcoming sub zero camping trip. Adding more is an option. You can also leaving running during the summer months to remove moisture.
2. Add a second heater to the basement area to make it an actual heated space if heating that space is important to you. For me I'm going to add insulation to the wall between the basement and living area.
3. Insulate your fresh water line.
Solutions for Heartland:
1. Add a vent to the rear underbelly area. The furnace appears to have space to add one more vent. You could drop it down under the fridge and into the rear underbelly.
2. increase the size of the current underbelly vent, extend and split it.