Matt,
Good observation on the GFI tripping, Chances of keeping the neutral load balanced with its source are pretty low.

If you need to run 2 A/C units from a 30 & 15Amp pedestal, you would be best to have one of the A/C units wired so you can plug it into an extension cord and directly to the 15Amp outlet on the pedestal. Then run the second A/C and everything else on the 30Amp the usual way through the coach wiring.