Dual Battery - Truck Question

[SUPER X]

I have a 97 PU 5.7 in a GMC.
Its setup with a isolator but when bought only had the one battery. When I bought the truck in 2012 it had this setup. I tested it and it works, but I do not have the extra battery in it.
I have two HD RV deepcells. Question can I use a deep cell as a the alternate battery off the isolator?
The truck is power nothing, Just PS/PB nothing else like windows, door locks. Its the 5thwheel puller.
Specs 100amp alternator and isolator.
I thought about a bigger alt, but then would need to upgrade the main alt to #4 wire, and the extra battery wires as they are not set to handle anything over maybe 125amp, plus replace the isolator with a heavier one. Trying to see if I ca get by with stock on hand so to speak.

Thanks

yes im doing it right now

 

[SUPER X]

The brakes are wired there too but a separate post, next to it. I cant recall if it had them when I got it. As two months after I got it, I had some fancy digital programmed thing put in that works great.
They truck is odd anyone, it has a transmission brake and a few other odds Ive not seen. My Father in law has the exact truck in 96, and not the Vortec FI like I do, but also he doesn't have some items I do on the truck. Yet his has power windows and doors LOL. Hell I don't know. LOL

It sounds like your truck might have been built as a service vehicle chassis . like the ones with the work utility box . With all the doors on the outside .I used to get old county and state trucks that would have dual batts trans brakes ect ect . The dual battery's could be for running overhead and work lights or electric winches ,tailgate lifts.small lift buckets , hydrophilic pumps ect. most would have all heavy duty stuff with the ability to hook up more and no soccer mom frills.The order could have changed and they just put a box on it or might have been ordered that way .