Several thoughts/suggestions may help decide?
One big issue is the batteries running down while stored so some thought is needed as it is not good for them ! But it is not as simple as we might like.
Step one is that the start battery has things like radio, ignition, etc that stay on when we shut it down.
Coach battery has some things which also stay on like CO and propane detector.
If we disconnect either or both, the equipment is not harmed but it may also mean you lose the radio presets if you have any. Some radios do, some don't but we never pay much attention to presets as we use the radio in different areas anyway!
Options:
1. Connect a trickle charger or plug the RV in but this requires you to watch to not let the water go low on the battery as that is a killer. Also letting the RV converter do the charge does NOT charge the start battery, only the coach, unless we set up a feature like a Trik-L-Start, etc. to let it do both.
But fully disconnecting the coach items left over doesn't have to be hard if we study a bit as there is likely to be a separate breaker to feed those small items, so tripping that breaker may appeal to you?
This snip of the drawing shows how the coach battery flows to the different breakers. Click the drawing for a better view, but the idea is that there is the green path which feeds through the disconnect to most things with the big breaker but the blue path goes around the disconnect to the small 6 amp breaker feeding the leftovers.
The drawing shows these breakers from the back side so if looking for that breaker next to the step, mentally turn it around as what shows on the right actually is the left!!
So if you find that 6 amp breaker as 4th from right when viewed from front, (the one between the 15 amp and 6 amp?) trip it and also switch the battery disconnect off, the coach battery load will be cut off!
But also keep in mind that batteries do "self-discharge" at some small pace and do need occasion looks to keep them in best shape!