ONAN 8500 TRIPS WHEN I TURN ON 1 AC UNIT

i think a weak breaker is less likely than the things i mentioned
although it is possible

50amp shore power is 50*120*2 = 12000 watts
50% more power than the generator

i think you have a problem on one genny line

two many items, or something that is failing,

Mr. Wizard Thank you going to rv tomorrow, will give it a shot how about a weak breaker on onan.

Thanks John

aux power when starting the Onan ?

if so sounds like a 'house battery' problem

only (1) particular a/c causes the Onan to trip ?

overload alarm or trip the breaker

could be the converter and the a/c are on the same generator leg (line 1 or line 2)

heavy charge load combined with a/c startup load 'bingo' too much load

and/or that a/c needs a new start capacitor for the compressor

start by checking the batteries, and cables, then turn off the circuit breaker for the converter and try to start that a/c