bren0035,
It seems to me that the guilty AC is more likely to severely drag line volts down and trip a breaker when it tries to start. This older unit may have taken more amps (especially starting) even when it was new, and now it has an old, dried out electrolytic start capacitor to boot. So the most likely unit to fail a smooth start SHOULD be the older one. Make sure it's not trying to immediately restart right after a shutdown when refrigerant pressure is not yet bled down.
You could check individual run amps when both are running and see if there is a difference. Individual start amps will be harder to catch than run amps.
If such checks reveal that the AC fails on start, you might consider installing an easy start capacitor on the compressor motor like some folks do when they have a smaller generator (like a Honda 2000) that can barely run AC, but not withstand the heavy start draw.
Here's a simple, short youtube install for example. Such caps are commonly available online just for RV's, just google "easy start capacitors".
Wes
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