Charging starting battery

There are several reasonable options. An AC powered maintainer/float charger is one reasonable one; however, do be aware that most need to be disconnected from the battery whenever they are disconnected from the AC power.

Another possible option is a solar maintainer; how possible or reasonable does very much depend on where you keep the RV, of course.

It may well be possible to activate the battery isolation relay (such as via the "emergency start" switch if you have one) to connect the batteries together. This would in my opinion be simpler and better than installing a duplicate switch to interconnect them.

One popular and effective solution is the Trik-L-Start, a gizmo that diverts a little of the house battery charge power to keep the starting battery charged. It's not too expensive, easy to install, and once installed needs no additional attention. This would be my first choice of all the options. (They also make a somewhat more powerful version called the Amp-L-Start, and a few different companies make other similar products...that usually are much more expensive while offering no great advantages that I can see.)

For about $25 they have a charge maintainer that can be mounted and plugged into your MH AC outlet. it is permanently connected to the start battery and maintains a charge. Amazon sells them. I use them on farm equipment.

charge maintainer