Coach battery wiring

[BobK]

My Greyhawk 31FK needs a new and larger coach battery so that I don't have to run the generator so much while boondocking. A larger or additional battery will not fit in the original Jayco location so I placed two Lifeline GPL 8DL 12 volt AGM batteries in the basement compartment at the back of the RV. Hooking the AC system was easy, but I'm not so sure about the 12 volt system.

At the original battery location a wire with blue tape leads from the positive post of the battery to some type of fuse/breaker, then to some type of electric canister. Could that be part of the coach battery shutoff that is switched just inside the RV, by the front door? From the electric canister a wire with blue tape seems to go to the 12 volt fuse box, again indicated by blue tape near that end of the wire. If this wire at the 12V fuse box is indeed the lead to the 12V system that is fine, but where/how do I hook the negative wire to the 12V system?