New to me... Nightmare?

My experience is anytime you buy any vehicle from someone else and they inform you of some 'little' problem, but try to assure you it's nothing big, it is a big issue. Otherwise previous owner would most likely have already fixed it. Always buyer beware. Hope your battery issue turns out to be minor. Personally, I would suspect there's more to it than simple cables. But, fixing the cables is the logical place to start. I'd be prepared for additional issues besides the cables. That's just me. There are times it helps to be an optimist and times to be a pessimist.
Personally, I always make 3 assumptions going into any 'project' or repair.
1. It will take longer than I think.
2. It will cost more than I think. Usually about twice my original estimate.
3. In the process, I'll find some problem I could not foresee that will have to be dealt with.
With these assumptions, trust me, it's easier to contend with the project or repair.

 

I think it was the "?" in the title of the post that threw me.

my bad

keep on, keeping on.

 

Batteries are $79 at Freightliner. New Mitsubishi starter is less than $400 at Freightliner. Just replaced mine in 1998 FLD120. I’ll dig out my receipt and give the exact price.

 

It’s very simple. Your Truck has the same ground set up as every other Freightliner. I forgot to mention the other frame ground at the starter. Clean them up. Problem solved.

 

That’s how mine is, now that I think about it. I gave 2 hots from batteries to starter.

 

Ehhh, he's already got the wire in his shop's junk closet...

 

The extra ground wire doesn't hurt anything, but you'd probably be in just as good a shape if you'd cleaned everything up, replaced the bad lugs, and replaced the one undersized wire from the negative battery post to the frame post. That frame rail can carry a LOT more than 290A assuming it doesn't have a major crack that has been JB Welded and painted over.

 

Start with cleaning up the the “glowing battery connector “ that’s where the problem most likely is.
A bad connection causes a high resistance which causes high heat at the connection which causes the connection to glow and be hot which causes low amperage at the starter which causes the starter to turn the engine over slow which causes the engine not to start.