My truck has two batts also, but I run a camper battery so I'm not pulling on them. That said, I only sit a day or two at the most, and I have yet to pull much at all out of my Group31 camper batt, even with heater, lights, fan, and some laptop charging. I think winter camping would use a lot more battery... I am currently recharging the camper batt through the 7-pin RV connector, though I added a relay underhood so the connector only gets power from the truck when it's running. Connecting them constantly will drain all three, and could allow them to drain each other if the truck sits for a long while.