[rames14]

First, you do not need to do anything to your 7 pin connector. That merely supplies a trickle charge, as mentioned above it provided around 7 amps at 12v. There are several different ways to do DC to DC charging. One is the Renogy system and their website will guide you to sizing. We have the BIM smart charger. It turns current flow to the battery on for 15 minutes and off for 15 minutes. I can get about 40 amps per hour of driving. So, with two 100ah batteries, five hours of driving would recharge you from zero. You will not regret lithium batteries. They are much lighter, maintain voltage as they discharge, and recharge faster than lead acid/AGM batteries. If you don’t already have one, spend the money on a good battery monitor like the Victron. YMMV, JMHO.