It seems odd to me that it is the 20A breaker in the RV that is tripping and not the household breaker. Also, is the 101V reading when the AC is drawing its surge load on startup or regen? Because if that's the case then 101V is fairly robust for a whopping draw like a 15k AC.
As others have said, the convereter may be spoiling the party by trying to recharge the batts while you start up the AC. But the heavy converter draw should go away in minutes, and it seems like the OP is saying that it takes a bit of time for the AC to "overheat" and trip the breaker.
50 ft of 10ga followed by 25ft of 6ga--I just don't see this as insufficient for running a 15k AC. Add to that maybe 100ft of 12ga line loss inside the house, and okay, I can see 101v on startup.
I've run a 13.5k AC for hours and hours on 100v or slightly less. Startup surge took the V into the low 90s. Yes, the AC will heat up, but all that does is shorten its life on the back end--these things just don't up and die because they spent a few hours at low voltage.
IIRC, UL aproval requires a device to work without malfunction down to 103V. Close enough for me!
