My microwave has an extension 120VAC circuit receptacle located at the top of my cabinet where the microwave is plugged into it...
You could unplug the microwave here and then plug your circuit tester into this 120VAC source and see what it reads... Might have a bad connection back to the trailer 120VAC Distribution center...
All of the grounds inside the trailer all routed to the 120VAC Power Distribution center and passed through back to the 120VAC ground wire from the source. This line is also going to frame ground on the trailer. The NEUTRAL Wire for all receptacle gets routed back to a terminal strip and is isolated from from the trailer ground and passes through to the shore power Neutral connection. The Circuit breakers are only in the HOT wire loop between the shore power HOT wire connection and each of the 120VAC HOT connection at the receptacles. I guess all of this makes sense haha...
This pictorial kinda shows what i am saying here...
Google Image
Roy Ken