I've used the Stanley FatMax BC8S smart charger/maintainer to keep the chassis battery charged while plugged in. Doesn't need any installation, or special wiring, so it can be easily moved from vehicle to vehicle, if desired.
To use it, plug your motorhome into shore power to "light up" the coach's AC wall outlets. Then plug the BC8S 110-120AC plug into one of them near an "always hot" 12V DC outlet, and plug the 12V DC cigar lighter adapter into the 12V DC "always hot" outlet, push the mode selector buttons for your particular requirements, and you're done. It has colored charging status lights to tell you what's happening. I have my RV/vehicles parked so that I can see the lights from the kitchen window, when we're not out on the road.
Here's where they can be purchased...
https://www.amazon.com/STANLEY-BC8S-Battery-Charger-Maintainer/dp/B00PZ0PMSA
I think Walmart also sells them for around $40.
