What is accepted battery draw down?

With no drain on them, they should be at 12.5 or 12.6, even after a week. If they are dead, even with the kill switch off, you have one that's shorted and killing the others.

Do you have a sulfur smell coming from them when the truck is running?

 

Load test the batteries.

I typically change them out every 2 years , before winter, 850CCA, keep them ‘tended’ 24/7/365.

If you have toasted lead blocks, change out alternator as well, keep it in the box for emergencies. Dead lead can kill the sparky whirly bird sometimes.

 

If the battery disconnect is wired correct you should have no drain with the switch off.

I suspect you have something wired direct to the batteries causing the drain.

 

With the kill switch off you should be good for over a month without issue.

Even without it turned off the batteries should *easily* last several days unless you have something like a chest freezer or household style fridge running.

 

We have farm equipment that will sit for 9 months with no issues. You have another draw or bad batteries

 

Simple way to check for a draw. Turn battery disconnect switch off. Pull negative cables off battery. Get test light and put it between positive battery post and the unhooked negative cable. If it lights up you have a draw.
I've done this with older cars. Test light lights up, then start pulling fuses till the light goes out. Boom found the draw

 

Easy to do on 42 year old vehicles with no electronics.

Not so much today.