"Dave" wrote

You could charge the battery with a solar
panel

That might not work so well in an application like this. If he has the 250
watt heater and runs it for 3 hours then he has removed 750 watt-hours of
energy from the battery (plus change).

A typical solar panel charger is meant for trickle charging and they only
put out something like 10 watts or less, although I've seen some up around
20 watts. That means that it will take 75 hours of charging time at 10
watts to replace that 750 watt-hours, and that's at 100% efficiency which,
as well all know, doesn't exist.

Most of these deep cycle cells are looking for a charge at around 20 amps
for 10 hours or so to bring them up to full charge, and then a float charge
to hold the terminal voltage around 13.8 volts.

Either a generator or a propane-fired pre-heater is the way to go here.

BDS