I tried the OP's idea way back as seen in that photo, because I wanted a back- up plan if the gen would not work.
In that photo posted earlier I was actually trying for a faster recharge with more amps, by adding the amps from the charger on the inverter- truck combo with the other chargers' amps being run from the Honda 3000 in the bed of the truck at the same time.
That worked until the amps tapered to where the Honda would run the total amps being accepted, and the amps from the extra charger on the inverter were not needed anymore. ( I did not have solar then or it would have added even more amps)
The OP's plan is not stupid. He just needs to get his spec numbers correct to make it work.
In my test, I ran the truck in idle for an hour, no problem. (I did not set fire to the wet grass with the cat cov either!) Adding some gas to get more revs did nothing at all to help make it work better.
ISTR trying it with the lights on too, but that sure didn't help--I read that somewhere, but think it must have been an idea from the old days before the newer style alternators and their voltage regulation.
As for efficiency--that means nothing! You are off-grid and just want to get by ok. So what if it costs something to get gas the next time you do a "fill-up" earlier than it would have been time for a fill-up?
If you want to save money, don't buy an RV. :)
