My experience with the Ford MH heater is that you will never get the same amount of heat, or at least think so, as you do your auto's. You need to remember that your autos have a much larger volume of air flow compared to area space being heated than the MH. Also, at least on out 31UL, the air vents are kinda on the small size being 2" dia.
We had ours on the other day when we started traveling and it took about 15 minutes to start feeling any change in temps. My biggest complaint with cold weather travels is the defroster, but that's another subject.