I don't like the snap in receptacles they used in my TH. I've been meaning to replace one that get regularly used by a 1500 watt heater with a residential receptacle. The wires only make tiny contact with the receptacle.
I also have a couple of spare breaker positions in my panel. I'm going to add a new breaker with one receptacle for a second heater. That will involve moving one circuit to the other side to keep both legs of the 50A balanced.
It's been on the list since I first got the rig but we've survived 3 winters without it. I expect to open up the back of those receptacles and find scorching.
One final thought - I'd be reluctant to plug a 1500W heater into a circuit with a 20A breaker. That might make the cord from your heater to the 20A extension a fuse.