Im not visualizing what you are saying very well.
If ALL of your batteries are in the bed, and there isnt one up front, then your best bet as far as a good ground wit low resistance would be to keep it very short and sweet, and ground it to the frame.
Now if you had a battery up front as well, you may want to run equal length runs to the front. This is what I have. I have one battery up front and two in back, 3 runs for power, 3 for ground.