There are combiners, which are really two or three diodes incased in a metal heat radiator. There are also units using FETs, more efficient, more expensive. The chassis battery can be hooked up in parallel with the house battery, anytime the house battery is charged, the chassis is charged too. Downside. The cheaper diode chargers drop a volt or so, so while the house batts get 13.5, the chassis gets 12.5. FET only drops a couple tenths, but is a lot more$$. Trickle chargers are a lot cheaper and simpler in the long run.
Jim and Valerie, 2005 Winnie Adventurer 37B, 2014 Subaru Forester Toad, hitches, brakes, anti-sways, autopilot, gourmet food on a Social Security budget.
"Wave as you go by."