Voltage drop is terrible after adding more batteries. :/

[mylows10 5,000+ posts]

Hey guys! So I just recently added four deka 105ah batteries and before that I only had my xs battery up front and a yellow top in the back to power an saz-2500 and a small infinity amp with my 140amp alternator.
I was originally dropping down to about 11.6-11.8

Now I'm dropping all the way down to 10.8-10.9.

Prior to putting the batteries in they were all given a full charge and load tested. Each battery was rated good.

Why am I dropping so low? Ideas? Suggestions?

Thanks!

Sounds like ones bad to me