Have just bought a 200w unit for use as an overnight heater, or at least a Pre heater.
It has a 3 hour timer fitted which is handy.
I have a twin battery setup so the idea is to set the fan up an hour before bed then leave it on for a little while after, maybe 30 mins (I have a bed setup in the car)
Obviously it is no problem to drain the secondary battery, I just hope that the heater will at least work at all to take the chill off, and that I will get a few hours without draining the battery completely flat, which is bad for battery health.
My battery monitors are visible from the bed so I will be able to keep tabs on what the situation is.
It hasn't arrived yet so will see later on whether it works at all (was 30 quid so I expect it to be half decent) and if it will suit my purpose.
My batteries are over 70ah I think so even at 20 amps it should manage 1.5 hours easy!