Flip the breaker off / on , some barely move when they trip .
Next would be break out your trusty multi-meter , with shore power disconnected , pull the breaker panel face plate off , and check that the set screw holding the wire in place; at the breaker; is tight , then hook up to power and check the breaker for 120 volt power at the wire.
If you have voltage coming out of the breaker , then you'll have to drop the A/C bezel and check power inside the unit.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.