I could see this working to keep a bed warm, but I'm a little skeptical that it's economically viable to give ambient heat to the vehicle space using any sort of electric heat.
I've had an idea for radiant heat mulling around in the back of my head. It wouldn't be feasible for a lot of people, but in a system with a propane fired water heater (anything with RV appliances), I wonder how efficient it would be to plumb a line up coiled between layers of flooring... something like van body, then insulation, barrier, PEX tubing, and a thin layer of flooring on top. The propane water heater would just run constantly when parked (thermostat would turn it on/off as needed) and a 12v pump would circulate hot water through the PEX to warm the floor.
Not sure how it'd work in application, but it's something I've been thinking about. It would use propane consistently but the only battery draw would be for a small circulating pump. Personally I'd rather fill propane tanks more often than run my batteries down, especially on cold rainy days.
Still just a thought and it still wouldn't work for anyone without the means to add RV style appliances.