Okay... Lots going on here...
First: Take a picture of your battery setup. Things should not melt when you hook them up unless something is very wrong. Most likely it is WAY before your starter.
Two: Slow down an tell us how yo got here... Did the truck ever drive? What changed? What did you do?
Three: more pictures. 24 volt stuff is easy if you know what you're looking for (and what to never do). Show us because we will see things you never thought to look at.
Start there and then we can ask the right questions...
OK. Photo tomorrow.. 8 months ago, I thought it ran perfect. I bought it, then had an emergency move after a house fire, so it sat for a while untouched, and there were 2 older batteries, but they appeared to be fin.e. It would start right up even on cold Buffalo morning. I went to run errands, 5 stops in 55 minutes, had heater on, lights, wipers, came out with dinner and didn't start: lights worked. AAA came, I informed them it's 24V, he tried to jump/replaced 1 battery, then towed me. Mechanic said I needed an alternator, and I believed him for a few months, couldn't find one, so brought it to a 85 year old Auto electric guy.. He tested it on ancient machines for 45 minutes, told me it was fine, most likely needed a voltage regulator, which I bought. I put alternator back in, then replaced the old battery with 1that matched the AAA one, and VERIFIED 24V hookup.. A lot of time passed, mechanic had it towed back to my house.. It was fully charged, and started up so I could park where I needed to. I moved it a week later to cut lawn, so if there was a drain, I couldn't do this.. But back to installing new battery. It seemed like a very excessive spark, this is when I verified 24V hookup(- from 1 to + on the other). When I tried, just touching the cable to post fused it in place, I knocked it off, wrapped the cable, and went inside.. So, what I know is there's way too much current/amps/watts, batteries were hooked up for a month while I looked for an alternator, and I moved car(started right up), and under full load with 5 stops, it wouldn't start sixth time.. Lastly, the old battery appears to have score marks on it from this sparking, so it's probably not a new problem, I just didn't know about it until that running those errands.. I can assure you, batteries are hooked up as 24V, not 12V.. When I bought it, the radio and lighter were missing, and the dash seemed pretty dark. Moving during Covid, it wasn't easy to just take it to a shop: there's no cabs or ride shares here. For the 10-20 minute rides I went on, it wasn't a big deal not to have a radio, getting stuff dry after the fire was a priority, and trying to unpack at my "project house".. Car was never near fire. While it seems possible this is a starter, it reminds me of a faulty ignition switch on a riding mower, fusing in 1 position.. I found my hobby test lights, but am not great with electricity.. A slow drain would've left me with a dead battery at some point, car would sit for 1-2 weeks at a time.. So, photos tomorrow. It's snowing here, so I'd like to do more than just disconnect the starter when I get all wrapped up.. Diesels are still pretty new to me too.. These things are built like mini tractor trailer semis. LOL.