Mine along the same lines as your van, my first battery is the primary and the second just feeds my cooling fans, and other equipment.
The wiring the two batteries are connected thru a 15Amp bounce resister thru NC contacts, and if the engine temp climbs the contacts open disconnecting the two batteries and another NO contacts close, connecting the one battery to the fans allowing the alternator to just be charge the primary battery as the fans run.
Then as the engine cools down the contacts again change reattaching the two batteries together, disconnecting the fans and the alternator is now recharging the cooling battery thru the bounce resister at a slower rate of only 15Amps allowing the alternator not to be overworked…
Note; I’m running an aftermarket 163 Amp Alternator, with two 800CCA batteries… B…