Measure the output voltage of the generator to assure proper voltage. Voltage may be low.
What is your altitude?
Plug a 1500w space heater into the generator, or similar load, and see how it operates on other loads.
I suspect the generator is lacking either voltage, fuel or air.
On Edit: if your camper has an electric water heater, use that as a load. Assure it has water in it. Plug your meter into an outlet inside the camper, start the generator. If voltage is not between 120 and 127VAC, Stop.
If voltage is in range, turn on the water heater.what's the voltage?