After 90 minutes (5:40 am) the Ambient temperature remains at -18 and the interior of the RV has reached 8 C (46 f).
It looks as if I timed this well so that my cleaning lady will be able to do her magic.
I'm "pushing" the Yamaha to the edge of its abilities. I do this by monitoring the charging voltage from the Magnum inverter--and the output voltage. When there is 13 volts of charging the alternating current is about 115 volts, I'm at the edge.
If I go "beyond" the capacity, then charging voltage drops to 12.2 (useless, at these temperatures). When that happens I reduce the load on the generator.