There are more btu's from the propane than from the 120 volt heater.
The heater may not be "seated" correctly.
Check to see how many watts it is drawing--it ought to be around about 325 watt.
It is also possible that the 120 voltage is on the low side. Power varies by the square of the voltage so:
120 x 120 = 14400
and
110 x 110 = 12100
That's 16% less power, so only 273 watts.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.