Yes I will be running two batteries under the bed.
You guys are right. I got them backwards.
I will be wiring the batteries in parallel. (That's what I meant

)
Wiring in series would indeed make the power 24 volts which I do not want.
I will be wiring the positive posts together and then just wire the negative posts to the frame.
I pretty much only buy from Summit Racing because I have an account with them. However I can not find the 1/0 cables I need.
I only need around 15' of positve cable and maybe only 3' of negative cable.
I found this one However the description doesn't seem to match the picture because it doesn't show the ground straps. I will also have to buy new post terminals because these are crimped on but it also show they are bare...? Maybe you get to pick...?
http://www.summitracing.com/parts/TAY-21540/
Brand Taylor Cable
Manufacturer's Part Number 21540
Part Type Battery Cables
Product Line Taylor Battery Cables
Summit Racing Part Number TAY-21540
UPC 088197215407
Battery Cable Type Positive or negative
Positive Cable Gauge 1/0-gauge
Negative Cable Gauge 1/0-gauge
Assembled No
Cable Jacket Material Silicone
Cable Jacket Finish Red
Cable Length (in) 240.000 in.
Battery Cable End Style 1 Top post
Battery Cable End Style 2 Bare wire
Adapters Included No
Hardware Included No
Auxiliary Lead No
Quantity Sold as a kit.
Notes Includes 2 eyelet and 2 top post terminals.
When resistance gets tough, the tough get Taylor battery cables. These 1-gauge cables are made from 980 strands of premium 30-gauge copper wire to deliver more cranking power with less voltage drop than most battery cables. They feature crimped tin plated brass post terminals that are less corrosive and three times more conducive than lead. Copper ring terminals for starter and ground connections are included, along with P-clip cable mounts and shrink sleeve tubing for a professional finish.