Howdy
Not sure about your truck...BUT...
In the early years of GMT400 and before, the main cab feed wire went to the starter and then back up to the load center on the passenger side firewall. It is beyond my understanding why it does this. I am sure that someone has a good reason
On three of my trucks I have removed this wire and ran a new heavy gauge wire from the alt to the batteries and then another wire from the batteries to the load center. Seems to work GREAT for me.
So, if it were me, I would start there.
Also install all new grounds from the batteries to the block, frame, and cab.
The main battery cables would also be suspect given the age of you truck.
Everyone's version of "fiddled with" is a little different
If you are going to keep the truck, I would replace all the battery cables with new, if it already hasn't been done.
If the dump is run by local municipality then it is your tax dollars paying for it so tell them people to back off or help as this your shop!
Good luck!
1996 Chvy 3500 CCDWLB 6.5 Heathed, NO vac, marine injctrs, ARB bumper, BIG pipe, 3" lift, bright lights, bypass oil system.
1986 Chvy 6.2 M1009 blazer RAM AIR, Headers, Custom interior
2001 Chinook RV, V10 gas
1974 John Deere 1530 diesel tractor
1993 John Deere 455 Diesel lawn mower
1967 GTO, 1989 Honda Transalp
2009 VW Jetta TDI, flashed and piped, 6speed, fun car!
1998 6.5 suburban, stock, daughter's
1993 6.5 3500 CCSWLB GM8, Heathed, big exhaust, gauges, Son's
1984 6.2 ATS turbo 3500, SCDWLB - Son's
3 kids, 1 wife, 1 dog
Gunsmith, Tactics Instructor, Fabricator USMC 87-93 Semper Fi!