Dead battery I think

You mean, won't turn over or spins but won't start. If it won't turn over with 12.8 volts, which is normal, might be a bad ground to the frame. Try the ground jumper( black) from your car right to the frame, see what happens. If it only clicks, bang on the starter with a hammer.

 

If you have the jumper cables hooked directly to the battery posts it will charge either way and it doesn’t matter.

 

Remember volts and AMPS are not the same thing.

It's amps that start the truck. Most trucks need a Minimum 1,000 amps to crank.

If you had to jump it 1st check and clean the batteries/cables. That may fix it. If not need to have the batteries "LOAD TESTED" 1-2 of them may be bad..

As to the battery kill switch it just kills power to the truck. it can be on or off when charging as long as your cables are hooked to the batteries

 

Sandpaper won't hurt it but a Wire Brush works better

 

Also check the terminals on the master switch to make sure they're tight & clean.

 

I always put additives in before fueling that helps mix them up well.
. Hot water will disolve that mess on the batteries, but always good to clean them up and check for proper securement.

 

If batteries are tired they are going to absolutely fail the load test.

Buy new batteries Plural for the entire group. If one or two of three or four are sick, the others are not far behind.

When you get into negative 30 degrees leave her running. A few gallons of fuel wont hurt her none.