Lmm not starting!

[DIESELMAFIAPER.LB7]

Replace the second bbattery you have a bad one

 

[NC-smokinlmm]

Replace the second bbattery you have a bad one

This^^^ load test the batteries...

 

[2001-GMC-LB7]

Also check alternator, if a diode shorted out it will kill the batteries!
I had one go bad on my truck and killed my batteries just like you are saying, if I drove it every day was no problem but leave it sit for a week and Dead Batteries...