Adding Solar Charger

You can add a trickle charger anywhere in any 12v circuit. It all ends up back at the battery.

You are right, it would have to be on the battery side of the disconnect switch.

if you use the D.S.
then Yes, the solar must be connected to the batteries,
to maintain charge while is storage, with batteries disconnected from RV

You can hook a "trickle charger" in to just about any circuit you want. There output is so small it won't matter much. Any circuit that stays live when you disconnect the battery switch will be fine.
Even a 10 watt solar panel puts out well under 1 amp.
In my opinion it's really not worth the effort. You might as well install a real solar panel, something like 100 watts minimum.