Microwave and AC Unit use 120V AC power to RUN........
AC Unit uses 12V DC for control functions but Microwave wouldn't need DC
I take it the '12 volt system' is actually 120V AC System ---Correct?
Which 120V C circuit breaker is tripping?
Is your 5th wheel 50A or is it a 30A rig.
4 prongs on power cord---50A
3 prongs on power cord---30A
IF 30A.....can't run microwave and AC at same time....even of plugged into a 50A power source using an adapter (30A to 50A)
If 50A.....should have been able to run both at same time. Unless AC Unit AND outlet Microwave is plugged into is being feed from SAME BUSS.