Driver slideout lights fuse keeps popping

[chrisan]

Got the RV back from service, working on the DC system (lots of debugging after installing solar & lithium battery upgrade). Everything seemed good.

Closed blinds, turned on AC, hooked up to 50A power. Sat for 2 days.

Tonight getting ready to leave in the morning... no lights in the driver slide. Hmm. Fuse is burnt. Replace (20A), lights good.

Come back later, fuse is burnt again. Replace, immediately pops. Cycle 12v system. Replace, immediately pops.

Lights not working isn't a huge deal, but the electric black out shades are down, including the driver side window, so it's not drivable. :(

Thoughts on how to debug?

[chrisan]

tried testing the circuit with voltmeter.

All the other DC 12v leads test out at 13.3 (ish)... the driver slide out lights read -20 (negative 20)...

If I pull a random fuse, the lead registers 0 (zero) as expected.

I think this means there is somehow a charge flowing in reverse from the lights or blinds?

[chrisan]

update: continuity tester says that there is a connection from the ground to the problematic out circuit... which I think means there is a ground fault... no idea how to debug that.