"Open Ground Hunt" was an unclear title, this is actually a "Hunt For Broken Current-Carrying Neutral" (AKA "Wight"). Wolf and MollysDad pointed out this stuff, but let me clarify a bit more:
- 120v "hot" black wires should only be connected to other 120v "hot" black wires, on their way back to a circuit breaker. (This part seems OK.)
- Current-Carrying "neutral" white wires should only be connected to other 120v "neutral" wires, on their way back to a shared "neutral" bus. This Bus connects the RV power cord "neutral". It does not connect to the "grounding" wires, or the frame, anywhere inside the Trailer.
- Green or bare "grounding" wires for 120v may be connected to each other, and also the the TraiIer frame, on their way back to a shared "grounding bus". The grounding bus connects to the green safety "grounding wire" on the RV power cord.
When outlets and appliances are operating correctly, The trailer frame and green wires should never carry current. (They carry current, and hopefully reduce all the Voltage, when appliances develop a "fault".) From the RV power cord, the "green" grounding wires should not be joined with "white" current carrying wires until they reach the Service Panel. (Green wires should also have a genuine "earth" grounding pole installed there.)
Your "broken path" for Current-Carrying Neutral might show a Voltage differential from the 120VAC hot lead (based on impedance), but can't carry current back to the Utility Service Panel.
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On NEMA 5-15R Receptacles, the narrow vertical is 120VAC "hot". The tall vertical is Current Carrying "Neutral". The "U"-shaped socket in the bottom middle is the safety "Grounding" terminal.
If you use a voltmeter to test this string of sockets, you should first test for Resistance - without
plugging the Trailer Power Cord into electrical Service. In order to do this, you will need a long piece of flexible, stranded wire.
First test, without the long cord:
Within each outlet, verify that Resistance between of the 3 terminals is infinity. (This probably passes, and it's not related to the situation you describe.)
Second test, with the long cord:
Connect one end of the long cord to the GROUNDING Bus (the one with green and/or naked wires attached). Verify that each of the "U"-shaped grounding terminals has ZERO resistance to the Bus. (This probably passes too, and it's not related to the problem you describe.)
But if you are checking these terminals, you might also want to verify that the frame has ZERO resistance to the Grounding Bus. There should be a big wire from a firmly connected frame lug into the Bus.
Third test, with the long cord:
Remove the end of the long cord from the GROUNDING Bus, and connect it to the Current-Carrying "NEUTRAL" Bus instead. Verify that the tall terminal on each socket, has ZERO resistance back to the Current Carrying Neutral Bus. (At one outlet or another, this test probably fails
. The box for the first failed outlet contains your broken wire path.)
If all tests pass, then the problem is not within the Trailer Body. Test the Power Cord.
Third Test, through the power cord:
Pull the end of the main Trailer Power Cord inside the Trailer, and set it down next to the power distribution panel. Check that all 3 plug ends (or, in the case of 50A service, all 4) have ZERO Resistance through the cable, and into their corresponding Distribution Panel Bus Bars.
For you, pay special attention to the Current-Carrying Neutral. If Resistance is not ZERO, then a wire is broken within the cable. (Replace the whole cable, do not try to repair it.)
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If all these tests pass, the the problem exists within your Service Outlet, and not within the Trailer at all.