GFCI tripping

[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 outlet is wired correctly.
On this circuit there should only be the circuit breaker, wire, and outlet. Should I assume the circuit breaker is the cause?

David and Julie
2015 CrossRoads Cruiser Aire 28SE
2011 GMC Sierra Duramax

[harley]

If you are electrically inclined. Simply go inside the converter and switch the breaker 15 amp breaker with another 15 amp breaker. Just swap the wires try that.

[Stan]

If your microwave is in a slide, it's possible you have water in a junction box under the slide out. Very common issue. Even if it's not in a slide, you might look for this junction box anyway.

[Masscamper]

Late update.

Tried all suggestions, must have had water in the junction box that is hidden and sealed. Could not prove but problem has fixed itself

[Lloyd]

Did you open the junction box to check for water or moisture? If that is the problem and it doesn't get sealed, you will have the same situation in the future.

[gregtx]

had the same problem. thought it was the wire from slide to frame. was gonna replace wire, when i took off cover on frame box, it was half full of water. no wonder it kept tripping. came for realize, they had installed box with self tapping screws through frame and into fresh water holding tank. i cleaned and got box dry, then sprayed several heavy coats of flex seal...... have not had a problem since.