Truck shuts off if you turn any of the accessories on

Just to clarify where are you getting these lower readings? On the clamps that are on the battery posts or at the other ends of the wiring?

The only reason you are going to lose voltage through wiring or connections is due to high resistance.

Resistance in the wiring acts like a load on the circuit and it requires current to get through the resistance. This causes the voltage to be lower.

Think of it as free flowing water and you put an obstruction in the water. It creates resistance in the water and requires work for the water to get past the obstruction and the amount of water on the other side of the obstruction is lowered.

This is the same thing with voltage. The amount of voltage needed to get through the higher resistance in the wiring takes away some of the voltage as it is used up.

We need to either do a voltage drop or resistance reading of the wiring to find out where the issue is.

Let me know if you have questions. Thanks

Just so I am clear. If you touch the actual posts you get one voltage. If you touch the clamps around the posts you get a lower voltage?

If this is the case, can you get pictures of the clamps on the posts? Make sure they are tight and clean. If you have build up on the clamps then that will cause resistance and steal voltage.

https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked

Okay, great. Glad you found it. Sorry if I wasn't clear but that is what I meant. Basically there is build up between the clamp and post creating almost like an insulation between the two. Normally it is on the clamp but clearly you found it. Thanks again.