Joseph, if you have isolated the issue to just two outlets then I doubt all that testing is needed.
If you get 90 volts on generator at those outlets and 120 volts on the other outlets I would say that indicates the issue is with the inverter circuit.
The only other test I can recommend is putting a meter on the output contacts of the inverter. That would tell you if its the inverter or something downstream. But anything downstream that I can think of would either be a wire break (no voltage at all) or a dead short (popped breaker). So while I would do that test, at this point I would say with 90% confidence it is the inverter going bad.
Do you know what model inverter you have on board?