I was reading an article on Exercising your generator, by RVlifestyles (also read the Onan manual) and it suggests using a ceramic heater for the suggested load when running the generator for 2 hrs at 50% load.
As long as the heater has a turnover switch, it should be safe on a counter or floor, if it is a safe distance from a flammable substance. I prefer the oscillating hot wire heater. Ceramic heaters get clogged with cat hair in our coach, as electric heaters tend to attract the cats. Now that we have a heat pump, we seldom use electric heaters.
Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow