A few years back I was having problems starting. I had 12+ at the batteries and less than 10 volts to the starter. I ended up replacing the battery cables to solve my problem. I up sized the cables from 2/0 to 4/0 when I pulled the new wires.

If you have 2 multi meters, put one on the batteries, one on the starter to see if you are losing voltage. If you done get voltage to the starter, put on meter at the disconnect switch to see if that is your problem.