Strange electrical problem

Self tapping screws are not the best. I would have used a bolt, star washer and nut. Clean frame good with wire brush? Coat the connection with electrical grease.

OK finally fixed. I just decided to install a new ground wire from the battery negative to the frame. Drilled a hole, used some #10 wire to a self tapping screw. Tested, now have 12 when off shore power.
That opens up boondocking ahead. What a pain tracking down these electrical problems can be. Fortunately the Mobile repair guy I called yesterday never called back so that saved us several hundred bucks anyway.
I can use that 20 feet of #4 wire to hook up the inverter when I get the solar system fully installed.
Thanks for all the suggestions and help. I can only wonder what happened to the original ground wire that worked for 2 years+

Put the test wire on the negative, then check for resistance between bare wire and it. You should be testing to the battery negative. The bare wire should be tying the 120 volt system to the chassis of your rig. Have you verified the battery negative connection at the battery?

You could get an extension cord. Use a clamp to touch a prong to the negative wire where it attaches to the battery.

Run the extension cord over to your panel and check for continuity between the cord and the negative at the panel.

OK an update, I identified the cable coming from the battery. Went out and bought 20 feet of No4 cable ran that outside the camper as a test from the 12V hot side at the batteries to the distribution block. Checked it against the bare ground wire at the dist. box end with 120V on, got 12V. Shut 120V off that same end now dead but it still has 12V at the battery end??!!
I'm beginning to think this may be a ground problem, what else could explain it?
Thinking about trying a No10 wire from the battery ground to the dist box, any thoughts on that?
There are NO fuses unless they are buried deep inside the camper belly.
BTW the original cable from the battery is distinctly smaller than the stuff I bought.

Quick look, your panel has two 12 volt DC inputs which means you should have two fuses.
Look at bottom of this for information.
Manual

Gary45 wrote:
Do you have fuses in battery location (Mine does) - one of the fuses will be the charging circuit

I'll double check that, I haven't seen any. The wiring just goes thru a wall w/o fuses. But there is a space under the camper where they come together, I'll open that up.

Do you have fuses in battery location (Mine does) - one of the fuses will be the charging circuit