Further updates:
I ordered a new FlowRite pump, as my old one was a little corroded. I'd thought it was still working fine, but based on how much water went in with the new pump, the old pump may have been far less effective than I'd thought. (My battery compartment has almost no extra space, so it's impossible to check on the levels.)
So the batteries are now full, though still old and possibly in bad shape. Last night, after filling, all the 12v lights continued to dim and flicker, though I haven't had any issues with the AC in a few days.
The Magnum remote continued to blink a fault of Overtemp or No AC In up until this morning, when it was stuck on "High ACV", though I believe I also saw AC Overload right before the High ACV.
I unplugged the shore power for 25 minutes and the fault didn't clear, so I reset the Magnum and topped off the batteries again. Then I plugged into the next spot's 30amp, as our neighbors just left. Same issues, so it doesn't look like there's an issue with the 30amp AC coming from the RV park.
Tested the batteries with my multimeter. I got readings of ~4.5-5v. The interior Magnum remote says ~9v. (This morning it was up around 12v, but it's been dropping.) EDIT: They're up to ~6.3v each and the remote is reading 12.7v. I believe the 4 batteries are 2 groups, so perhaps that's about what they should be, and indicates the interior reading is accurate.
Right now, the Magnum is sort kicking on and off the fan ever second or so. 1s on, 1s off. Was doing that for 15/20min.
My current plan is: Get new batteries this evening and install them tonight or tomorrow. If the problem goes away, it was the batteries. If the new batteries have the same issue, I'll swap the old back in and look at getting a pro to diagnose or replace the Magnum. But the batteries are 6yo anyway, so probably wouldn't hurt to get new ones.
Also, I don't think my Magnum is a converter. Only an inverter and charger. I'm not sure if there's a converter elsewhere on the rig, but based on the battery voltage and the state of my 12v lights, my hunch is that my rig simply needs the batteries to have any 12v power. I'll confirm that when I take the batteries out, I suppose, and I'll update here.
UPDATE: For the past few hours, the Magnum's been charging fine, and the batteries are now ~12.7v. I'm going to delay on the batteries until tomorrow, to see what another day looks like.
