I'd jump into one of the AC threads and ask the experts - the fact that your AC unit appears to be sharing one start cap for double duty could be your problem - I know mine had 2, and I only piggybacked on the start capacitor.
I'm not an electrical tech (at least 120V, I can do 12V stuff in my sleep) so what's what under the hood of the AC isn't exactly clear to me, but the guys in the AC threads will be able to tell you for sure.
Just be prepared to ignore the "Oh geez, sell those pieces of junk and go buy a Honda/Yamaha" people who rush to the conclusion that it's the generators fault, not the AC. There are lots of people here who are quick to spend other people's money.
Solving your problem could be as easy as adding a second capacitor somehow. I suspect that the second trailer you tried had the 2 capacitors. which is why you were able to start the AC on it....probably even without the SPP6 upgrade, which would then likely have allowed you to start it on Eco mode. ;)
