Of course the amount of furnace AH used depends on the RV. In the 5er barn it was a lot. 35F ambient. In the MH we have now, not so much.
30% charging rate from 50% SOC will not boil the batteries at all. In fact you get to Vabs at 60.X % SOC and then amps taper. The trick is to restore enough AH in a 50-80 or 50-90 to get by till next day's recharge.
You have to juggle AH use in a day vs how big a battery bank your RV can carry, vs how big a charger that needs, vs how big of a generator can you carry. We got away with it with the 5er because the Honda 3000 went in the truck. With the MH (see photo in Profile) no room for that big Honda (and no built- in gen), but also it is easier to heat.
You do what you have to do.
