Update: flap in the foam around the water heater?

I think you have a good idea. It would save the main batteries and probably be more cost effective than a Buddy type heater. Also I would consider cutting a 4 in hole in the dinette seat vertical wall and installing a computer fan to distribute the heat once you have determined enough heat is coming off of the HWH. Just be aware that you are not getting free heat but the savings will be in battery power and using a cheaper source of propane than those little bottles.