There isn't much more you can do about it tonight, but if it were me, tomorrow I would go ahead and pull the breaker and hit the home improvement store. If you have a GFI on that circuit, I would pick up a new GFI, breaker, and if you don't have one yet, an inexpensive voltmeter.
In turn, I would switch them out and see if that resolves the problem. Worst case, you end up with a spare something or other that would be good to have around anyway.. and if it turns out NOT to be the GFI, then the new one is more than likely going to be of higher quality than the one you pulled. All told it shouldn't be more than $20. Less if there is no GFI on that circuit.
Good news though!! It's NOT your cord, power supply, transfer switch or converter.
If a breaker and GFI switch out doesn't cure the problem, then I would first pull all of the outlets.. starting with the one the electric heater was plugged into and inspect for damage. If there is nothing obvious there then you'll need to do a continuity test along each leg of the circuit.
If you DO end up having to run new lines it's generally not that big of a deal. Just attach the new line to the end of the old line and then feed/pull it back through from outlet to outlet.
Hope it works out.