Are you sure you have 4 12 volt? I'll bet you have 4 6 volts.
Anywhere but CW. Get some quality DEEP cycle from Costco, Sam's, NAPA or such. They really aren't hard to change. Take a picture of the way they are wired, mark them or both.
The 12volt Side of Life (Part 1) Has some good information. Then install a battery water system such as
https://flow-rite.com/battery-watering/marine-rv you can find these online from $40 up.
Find someone in the area to weld the rack, knowing that you will have to remove the batteries first.
Hard to beat the Trojan line for quality.
Trojan Battery Company Dealers all over.