My neighbor has a '95 Chevy truck 6.5 turbo diesel. The electrical system is messed up... he just replaced alternator twice and the dual batteries because it drained but the voltage gauge still reads low and he's afraid it will die again soon. I told him to try disconnecting the batteries with it running and sure enough it died like the alternator's bad.

Anybody know these things or a trustworthy mechanic. I don't feel like digging into his wiring.

Check the ground cables.

time to buy it from him.. and use it as a tow vehicle.