Only one fuel gauge is connected through at a time. If the valve is working then that's unlikely to be the problem on both tanks.
The bulkhead plug is the big square Packard plug that most of the wires pass through into the cab. It's near the booster.
The 16ga pink wire next to the light blue wire at the bottom left on the pictured bulkhead plug is Circuit 30.
Circuit 30 is the GM name for the fuel gauge wire from the frame disconnect that plugs into the valve sub harness wire to valve terminal B.
The bulkhead plug is shown in the first full page of wiring in the 86 wiring diagrams.
You can backprobe the bulkhead plug with a T-pin to hook up your meter. OTOH disconnecting the bulkhead plug isn't rocket science. Unscrew the bolt and the plug comes apart like Lego blocks. Unplug the battery before you disconnect the bulkhead plug.
I'd take a meter and check for continuity from the bulkhead plug to terminal B on the valve plug. If you don't have continuity you've found your problem area.
It's usually chewed up down under the cab near the valve subharness disconnect.