It's unfortunate that your model of Yamaha can't be paralleled; that would provide any easy solution.
However, I think that with a little work you should be able to start your AC unit. Is the "new rig" really new or a used one? It's possible that the AC unit is wearing out: bad bearings, weak capacitor, low on oil, etc. That makes it hard to start.
Have you tried it with the Yamaha running full speed and not throttled back? That may make enough of a difference. The hard start capacitor should also make an improvement. Do you have a long extension cord between the RV and gen.? That would make it more difficult also.
