Just throwing out ideas here. It sounds like you've pretty well eliminated the external electrical hookup as a cause. Makes me think there 3 possible big-picture things to suspect:
1) Engine power problem
2) Internal electrical problem
3) Generator specs are overstated
Other people can address the engine issues better than I. To see if the specs are overstated, it would be nice to have another unit for comparison, or a good load test performed by somebody who has another copy of the same model. On the internal electrical side, I'd first ask if one or both of the output windings might not be supplying its share of the power due to (1) low output, or (2) wiring error between the winding and outlets. If for instance there was a wiring error at the voltage selector switch, the entire load could be carried by only one of the two generator output windings. An easy way to check the individual windings would be to run in 240 volt mode and cobble up a plug+cord so you can run each heater off one of the 240 volt legs, and take measurements. You could drop the control panel and inspect and ohm out the current paths from the winding side of the switch to the outlets. It sure would be nice to know up front if you really have a weak unit vs. overrated specs. Interestingly, the manual I downloaded from the Champion site doesn't mention the existence of the voltage selector switch except in the parts list. It's not even shown on the Power Panel illustration.
