You need a charge solenoid or relay. If you're going to solar, I'd recommend a relay that will allow the chassis battery to also be charged from the solar.
This is the one I use and recommend:
Automatically combines batteries during charging, isolates batteries when discharging and when starting engines.
www.bluesea.com
It's about $85
You'll need to add a fuse to the positive cable between the starting/chassis battery and the relay, and also to the cable between the relay and the house battery.
This is how I did it here:
Things are moving along on the 03 158" cargo conversion. First thing I did was install an aux battery system using the BlueSea SI-Series Automatic Charging Relay #7610 and a Cabela's Advanced Angler AGM Battery. Hooked up the center console, radio, cargo lights to the aux battery and also...
sprinter-source.com
The relay also connects to the starting circuit to decouple the house battery from the chassis when you start the engine