Grab a few space heaters. Set 'em on LOW and make sure no two of them are on the same circuit breaker. 3 of them will be about 55%
If your rig is wired for a washing machine use that outlet for one of them. That one may be a 20 amp breaker if it is you can go to HIGH on that one and one more
Other wise keep 'em on low. and as I said Spread the load across 3 breakers
Don't have that many outlet circuits. Well two may be enough to get the heat up on the genny.
Re the Pure gas suggestion
There are TWO reasons to run a generator often ONE is gasoline, the other is electronic.