A very likely possibility is the plug in receptacle that you use is robbing the circuit. Typically in RV's they use plug in wiring. What that means is they push in the wire, and two skinny blades make contact with the bare wire. There is not enough connection to the wire to actually carry the load properly to the heater, so the connection heats up a bit and uses part of the wattage in the circuit, and causes the breaker to trip.
If you can change out the receptacle that you plug into, and put in a proper residential receptacle where the wires wrap around a terminal that might help.
If you can get an electrician with a clip on ammeter to check your circuit loads, that will likely find your problem.