Electric heat is 100% efficient, and 1500 Watts is 1500 Watts. Anything else is just fancy overpriced packaging. The little $20 ceramic cube heater produces the exact same amount of heat as the $300 Amish hand-crafted wooden heaters, or the $100 things that look like radiators.
Each 1500W heater draws about 14 Amps, so you will be overloading a 30A RV circuit trying to run two of them, along with charging batteries, running the furnace, etc.. Forget plugging them both into the same outlet, you'll blow the camper's internal breaker on that circuit.
20C is 68F! Pretty balmy considering you're in a box with the insulation R-value of a shopping bag, and it's -40C/-40F outside!
All you need is a fan to circulate that bubble of warm air around the camper.