Let's take a little different angle on the discussion:
What don't you like about the heaters you've tried?
If they don't keep the camper warm enough, go back to what others have said previously: All electric heaters are 100% efficient. Watts = BTUs. One 1500W heater isn't going to produce more heat than any other 1500W heater.
A single heater just won't produce enough BTUs to keep the camper comfortably warm if it's that cold. Two heaters will overload a single circuit in your camper, and if you do happen to connect them to separate circuits, you're pretty much maxxed out on the 30A shore power, especially if the converter is charging the house battery.
Now there are some creative solutions for this. I personally would take my 50A-to-twin-30A splitter out to the power post, feed the two 30A receptacles into the camper, and run a heater off each leg.