I have a Northern Lite 9.6 – might be a similar configuration. I upgraded to lithium, and installed the batteries and inverter under the dinette seat, leaving most of the wiring as-is in the original battery compartment. I had on hand some old #10 cable, 4 conductor, so I connected 2 of the four together each for power and ground and ran that from the new battery location to the original compartment. I used roughly 14 feet of cable. The biggest draw in my system is the inverter, which is mounted next to the new batteries. The biggest draw running through the old battery compartment would be the electric jacks, if I run all four at the same time, roughly 30 amps. I usually just operate two at a time.
Incidentally, I also have the Renogy 20A DC-DC charger and I de-rated it to 10 amps. At 20 amps output it was drawing 30 amps through my truck wiring and umbilical. My to-do list includes upgrading my truck-side wiring to handle more current, then I can run the DC-DC at full power.