My 8.1l has two batteries. The truck is dead after sitting for a day. I`m not sure how the batteries are wired.. Do they both start the truck in series?
Guess I could get them both load tested and figure out if I need to replace just one or both..

They are wired parallel. The cable from the passenger side battery comes over to the main battery on the driver side. Check the ground on the driver side battery to the block. I've seen 2 rot off.

If one is weak it will take the other one down over time. I would have both batteries tested (you will have to unhook for accurate reading) and if they are ok you have an open draw somewhere.....which is always fun to track down.

Weak batteries? Time for a new duramax!! Haha good luck Dan. How old are the batteries?

I had a bad blower motor resistor that was draining my dual batteries on my old 2001 8.1 2500hd.

How the heck did you/someone find that? Or is it common?

Batteries, alternators, starters...
Sometimes the symptoms can point to one of those 3, but actually be another of the three. One of those bad can also kill the other two.

Don't have Advance Auto do the load test in the parking lot. It's really not a good test of anything.
Pull them out, and take them to Sears. They have a diagnostic station they call the "microwave" as that's what it looks like. The batteries go in there for 10 minutes for a full test (and hyper-charge if good). They do charge for this, but it's worthwhile IMHO.

FWIW, Sears Die-Hard Gold in everything I own with tires.

It's common, there not actually a resistor. They are a pulse module. It pulses the ground to control speed, and gets stuck on. Blower just keeps going even with the key off.

Last time I jumped it, the HVAC blew full on heat on the drivers side and full ac on the passenger side.lol Good thing it fixed itself once I shut the truck off.

The batteries are 9 years old and I think this winter finished them off as the truck sits all winter.

thanx for the advice .. I`ll check that ground

9 yrs OLD! lucky You I would say you got your moneys worth. My 01 8.1 needed them about every 5 yrs. Didn't use much so probably didn't help Batt life.

Blower motor would not hardly turn off or change fan speeds. When I could get it to shut off, sometimes the blower just come on during the night and drain the battery.
For my current 2007 Classic 2500HD, they sent me out a bulletin that some also have issues and they will replace it up to 150k miles.