mvbn1:I have two deep cycle batteries mounted in the rear of the van, on the driver's side, forward of the wheelwell. I used a very flexible #6 welders cable I purchased at a welder supply shop. I taped two lengths of the cable together, side by side with 3M 33+ electrical tape and fastened copper eyelets to each of the ends of this bundle. I marked the pos/neg at each end to avoid later confusion.
With the wire bundle completed, I secured one end directly to the primary battery. I used the ground thru the primary battery rather than ground in the rear so that I would have a good sound connection. I fed this from the battery, down alond the frame to the rear and passed it thru a hole, made with a greenlee punch, into the cargo area I clamped this wire at every available point along the chasis.
The pos side of the bundle it fastened to the input of a continuous duty solenoid (carquest). another cable section runs from the output side of the solenoid to the pos terminal on the deep cycle battery. The neg side of the bundle goes to the neg side of the other deep cycle battery. Then connect the pos/pos and neg/neg of each deep cycle battery. At this point you have three connected batteries that will charge and discharge simultaniously.
In order to open/close the solenoid, attach a connection from an ignition controlled source, such as the electric windows, and you'll effectively, isolate the deep cycles from the primary when the ignition is off. You'll charge while engine running. I've used this method with success on three different vans.
good luck