Hi,
The largest cheap heat system would be sufficient down to about -27 C (-17 f) in my class C. However, because that would require a fifty amp service conversion, it is not possible for me to install it, without a lot of additional expense. Below those temperatures, it would be necessary to change over propane as the system design does not allow for "series" operation. (i.e. it is either electric, or propane, not both at once).
I am heating 100% electrically and have used as much as 5800 watts. So far the low has been -34 c (-30 f). I do this by having auxiliary shore power cords, and a 50 amp break out box. I can get to 7800 watts before the system is totally maxed out. However, it is prudent for continuous use to keep at 80% of that number or 6280 watts. That means I have a bit of "wiggle" room left.
My system is not for every one--but it does allow propane and electric heat to be used together.
Once my wattage use drops to 2880, I'll be going back to urban boondocking.