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 u...
So one of the main things a GFCI does, is measure that all current going out the hot is returned on the common neutral. So somewhere in the MW circuit it is leaking current on the hot lead. As you stated that the minute you turn on that breaker, the GFCI trips even without the microwave plugged. Moisture in a slide junction box or cable comes to mind.