Ground fault when I plug into 120 v outlet

You must unplug items to isolate. Breaker alone can still allow GF tripping.

NO problem with multiple GFCI's in series I did it often in my Sticks and Bricks days.

The problem is likely the converter/inverter, Some of them do not play nice with "Up-Stream" GFCI's but other suspects include fridge and water heater.

Basically it's telling you there IS a ground fault.

ScottG wrote:
You must unplug items to isolate. Breaker alone can still allow GF tripping.

X2

If the white and bare copper wires ever come in contact with each other (in the panel, in the wires or within any appliance that’s plugged in), the GFCI will trip. You have to physically disconnect the neutral wire in the panel to truly isolate the branch circuits. That’s why unplugging the fridge or other appliances is easier.
Sometimes the bare copper ground wire can shift within the receptacle box and bump in to the neutral screw.

Just de-winterized the rv before this trip. Turned on the electric water heater. After 30 minutes or so my wife says, still no hot water. SLAP IN THE FOREHEAD MOMENT. You guessed it. I forgot to take the heater out of bypass mode. Switched to gas. Went down and disconnected the white wire to check for continuity. Don't know why. I knew it was open.

I'm thinking, the water heater element could not possibly cause the ground fault, since it was turned off in two places and the white wire was disconnected. I just proved to myself again that there are (almost) no such thing as coincidences. This could not be related to the ground fault problem.

Wrong!! Leaving the black wire attached to the grounded heater element created the ground fault.

Wnjj: you are on the right path.

I learned the lesson about the heater element and the empty water heater about 20 years ago. If this lesson lasts another 20 years I'll be 95 and probably be eating spoon fed pablum.

I once had this same problem that was near impossible to determine the cause.... After MUCH trial and error experimentation, I finally disconnected all the neutrals in the main panel, then reconnected them one by one until the GFI tripped again...Turned out to be a leaky heating element in the hot water heater - a trace amount of current was passing thru the waters mineral content...

Hope this helps,

3 tons

Turn off all breakers in RV and plug in. Does it still trip?

NO - One of those circuits is the culprit. Turn them back on one at a time till you find the one that trips it. Trace that circuit.

YES - You have a NEUTRAL/GROUND connection somewhere. Disconnect every NEUTRAL wire in the breaker box. Reconnect them one at a time until you find the one that trips it. Trace that circuit.

If it still trips even with all breakers off and all NEUTRAL wires disconnected, the problem is before the breaker box. Trace that circuit.

As it happened, I knew I had a bad electrical element in the water heater. I began taking care of that problem first. After that correction the GFI didn't trip again. Problem solved. As an afterthought I realized why.

Rule #1, to me, is "If two or more problems show up at the same time, always assume, first, that they are related". During my working career I managed up to 90 maintenance people and always preached this rule. It almost always holds true and saves a lot of time.

Or, fix the stuff you know is wrong and all the weird stuff will go away. IMO

Disconnecting the NEUTRAL wires would have led you straight back to the water heater. The NEUTRAL wire side of the heating element was grounded out. Glad you got it fixed with a minimum of effort and expense. These gremlins can be a bear to find.