Is it possible to plug a different MICROWAVE unit into this plug and see what it does... I had a microwave unit that for some reason really tasked the 120VAC line. It worked just fine but sometimes it trip the breaker and also made a louder sound when operating than similar microwave units would. Somehow it modulated the 120VAC line differently so to speak when being used...
We all are just plugging away with things to check - glad you taking it all in... Hard to troubleshoot things without actually being there haha...
If the tester is not showing any faults with your 120VAC receptacle where the microwave plugs into it then I would go with the circuit tester. Perhaps something is weird about the microwave unit... You are definitely doing the things I would be doing tracking down the problem...
Have you ever seen the microwave work before using a regular RV 30AMP Pedestal type connection...
I know my microwave in my 5th wheel is also an convection oven combo which I have never used that part of it... It really sounds weird when running in convection mode...
In my off-road POPUP trailer which is what I only camp in now I have used both a high wattage and low wattage manual type microwave units without any 120VAC problems... When camping off-grid I use the low wattage white faced microwave with spring loaded dial-up timer hehe... It draws around 600 watts... I only use it for heating up my coffee and doing a 5-minute baked potato wrapped in cellophane to go with my steaks on the grill...
Roy Ken