I haven't used one, but my nephew has been running this heater for a few years without issue in a 7x14 cargo conversion.
Flash sale has it at $79.99 with free shipping in the US until Monday.
$65.99 if you're on their email list.
https://www.banggood.com/HCalory-12V-2KW-or-5-8KW-Car-Parking-Heater-Air-Diesels-Remote-Control-LCD-Monitor-for-Car-RV-Motor-home-Trailer-Trucks-Boats-p-1809016.html?cur_warehouse=USA&ID=6302999