question

With 114V open circuit, is the 74V also open circuit measurement?
If there are no problems along the cable, then you would also get 114V at the MPPT end open circuit - i.e. no current is flowing, so the Voltage drop should be very close to 0V.

Yes, I measure 114v open circuit at the panels and 74v open circuit right before the charger. I guess that makes sense that there shouldn't be voltage drop if there's no current - I suppose an animal could have chewed and made some sort of short where it's buried? Could that explain the voltage drop? It seems a bit implausible that it would chew both cables in the same place, and then they would also be contacting each other, so I just want to rule anything else out before I have to dig up all my cables.

@WKirby

Okay, I disconnected the PV wires from everything and read 120v open circuit there, so I assume the panels are fine. Then, I did two resistance tests at inside my house for my three cables leading to the panel - one where the lines are all tied together and isolated right before the panel, and one where they are all floating there.

Three lines tied together:

+ cable to - cable: Mohm level resistance

- cable to spare cable: virtually no resistance

+ cable to spare cable: Mohm level resistance

Three lines floating:

+ cable to - cable: 60k ohm resistance

- cable to spare cable: 18k ohm resistance

+ cable to spare cable: 8Mohm resistance

Does any insight about what this means or how my cables could have suddenly turned into resistors if these values are abnormal? Is there a short or is this normal resistance? Could this have something to do with a ground loop or something?