IME RV heat pumps are not worth much below around 10C. They’re okay to take some chill out of the air in temperate weather. The propane/diesel furnace is a much better option for heating below 10C.
What's special about RV heat pumps which makes them so bad compared to regular heat pumps? Heat pumps have gotten significantly better lately, and house-models are effecient down to -25C. Maybe the RV models you have tried are mostly older models?
RV heat pump are not that sophisticated because there doesn't seem to be much demand, they simply flip over to furnace. They are the same on the cooling side, they are no where near as advanced as a modern variable speed mini split.
There is no reason they could be more advanced with lower temperature support, they would just cost more. Many people put mini splits when making their own RV's because they work better and want to run off battery as much as possible.
Running that heater for 24 hours a day would take 12kWh - or about 1/6 of the battery in an electric van.
That seems feasible to me. Sure, it's a bit annoying - you have to charge more often - but you could get through a couple of days off grid.