Ok so I found the spot to tie into, the second alternator is wired through a 175amp fuse that is attached to a buss bar on the aux battery. There is a 3/8" stud just past the 175amp fuse, I will install a 120amp breaker and a automatic charging relay by the battery.
I am looking at the 4 AWG wire at the link you posted, any advice on connectors and a crimping tool? I want to have a charging circuit in the back seat area and also have a plug mounted in the bed to hook up to the camper batteries, would I be better off running a dedicated line to each spot from the charging relay or splitting the wire under the cab?
ETA
Also you say "with chassis negative return" what does that mean? Do I run the positive from the engine bay and then just bond the negative to the chassis close to the battery bank? I was thinking about running the negative from the main post in the engine bay, would this affect my wire sizing?