I'd be looking at options that don't require any power at all. I know your space is limited and something like this would be ridiculous overkill, but this is an example. There are probably smaller options.
Fuel is gravity fed, ventilation via chimney and stack effect, no electricity needed.
Dickinson 00-ALA Diesel Alaska Floor Mount Heater - Diesel Fireplace
www.suremarineservice.com
Fuel is gravity fed, ventilation via chimney and stack effect, no electricity needed.