We run our furnaces on the road whenever it gets cold the dash air doesn't do much by itself.
And if you have enough battery capacity run it at night if not plugged in.
We only have four house batteries and have run the furnaces off them several nights. Check your manual. All that runs off the batteries are the blower motors and thermostat, if that..plus the usual fridge fan even on propane etc.
In 2010 I think, we came back from Disney World and it was not higher than twenty degrees the whole way. we ran the furnaces and like to have roasted as the switch to the rear furnace the contacts stuck, so we seated then froze then sweated and froze. e also had a big hole under the shower ptu there by the dealer. Factory repaired it and both furnaces running it is nice at any temp outside and the water tank get heat too.