GFCI

[Masscamper]

We have just brought home (new to us) a 2015 Cruiser Aire fifth wheel. Plugged it into our outside 20A outlet and it trips the GFCI. I have isolated the problem to the 15A microwave circuit. I have unplugged the microwave and used a circuit tester to verify that it wired correctly.
On this circuit there is only the circuit breaker, wire, and outlet. Should I assume the circuit breaker is the cause?

[CA_Traveler]

Use an extension cord to connect the MW to your GFCI outside plug. Results?

[ScottG]

Have you unplugged the micro? It can cause a ground fault.
A standard breaker isn't able to create a ground fault by itself.

[Masscamper]

Yes, I have tested the source outlet with that same circuit tester and the two bottom yellow lights lit up verifying the outlet is wired correctly.

If I leave the microwave circuit breaker off, it does not trip.

[RoyB]

Have you also checked the 20A outlet source with your circuit tester before you have plugged in the trailer Shore Power cable or RV30A to 15A long adapter.... You want to see the two bottom lights burning for CORRECT from the source 120VAC as shown in this photo...

Google Image

Most of the time the GFCI is tripped because of a bad ground wire on the source connection in your House/Garage 120VAC connection.

Also the RV30A to 15A long adapter may be at fault with its ground pass through. If you are using an extension cord plugged into the 120V outlet before using the RV30A to 15A Long Adapter and your Shore power cable you need to check the extension cord as well with the circuit tester.

Anything you plug into between the 120VAC source connection could be at fault before you plug-in the trailer Shore Power cable connector which could trip the GFCI breaker.

Roy Ken