Slow Cranking Truck wont start

So this is my spare truck, International Eagle 9200, with Cummins ISX signature pre EGR, I am not using this truck at the moment so usually I turn it over and let it run once a week and take it for a drive every 3-4 weeks.

Over the last 2 years, every now and then when you hit the ignition nothing happens, just a small click, but then you hit the key again and it starts fine, I dont think this is the key ignition barrel as that is brand new.

I took the truck for a drive about 3 weeks ago, and then since then it has been cranking slower and slower each time but firing, until this week when it cranks really slow and wont start.

I have tested the batteries under load and they seem to hold up fine, based on my cheaper voltage meter, However when you test it at the starter I get 13 volts while resting, but when it cranks it drops to about 6-8 volts, I am not sure if this is normal? The starter gets super hot immdeiately, like burn your finger hot. The other day I thought I smelt a burning electrical smell, but that now seems to have gone.

When I put my jump starter on the batteries it comes up with an error indicating a short somewhere, I have taken all of the cables off, cleaned the connections and cleaned the ground connections on the frame. Maybe a short in the starter? As I dont think there is anything wrong with the cabling.

I am thinking it maybe the starter? Does the above indicate this?

If it is the batteries, Whats the best way to jump it? Are you better off jumping it directly at the starter with jumper cables or onto another battery? Its got 4 batteries, can I hook a couple of cars to the multiple batteries to give it a boost?

Any suggestions would be appreciated

Thanks

 

I tested each battery under load, about 13 volts normal, 12v under load.

At the starter, it holds under load at 12.5v

Whats the best way to turn an ISX over by hand? How easy or hard should it be?

I am starting to think the worse, I have noticed a little bit of coolant loss while the truck is resting.

 

I tested each battery with a load tester, and I tested the terminals on the starter with a load tester, All seemed ok

 

So, I took 4 batteries out of another truck that are only 1 month old, I put the batteries from the non starter truck into my second truck, second truck had no issues starting.

Still no start with second set of batteries. So I am 99.99% sure it's not the batteries

It seems to me as if the truck is only trying to start off 2 batteries instead of 4, so maybe there is a bad connection on one of the cables.

It does crank better after putting the batteries on the charge, but not enough to start it

I put a bar on the alternator and turned the engine over by hand, and it turns over easily (hope I did this right)

So I am thinking bad connection of bad starter.