I have a very old 800 watt wall heater that I fixed to a frame I welded up with wheels.
The kids each had one in their bedrooms in Johannesburg some 28 years ago. (The manufacturer stopped making them some 15 before that.)
I added a froststat thermostat to both and use one in my swb Boxer van
https://www.ebay.co.uk/itm/Honeywell-Froststat/164432903905?hash=item2648f6fee1:g:tX4AAOSw741ff~0pI set it so it kicks on at about 3 degrees and switches off at around 6 degrees. I have never had a hose reel freeze up, even in the bad winter in 2010/11 that hit us bad in the east coast.
I used to also fill up at night in winter. Even with cold water in the IBC tank in the garage at 5 or 6 degrees that water in the tank does also behave like a giant heat source. I used to throw a large duvet over the tank which also covered the pumps on one side and hose reels on the other. I can't do that now as I have a pair of step ladders mounted to the tank frame.
The only issue I have had with a van mount in 14 years was that the van ports froze on the way to the first clean. I went home.