"What he was proposing is very often done on cars for storage
etc, except of course there is no battery switch there."
There is, sorta. A lot of cars have the ciggy lighters go dead when the ignition has been turned off, so the lighter sockets are physically cut off from the battery, although they are still connected to random other devices.
Some solar "maintainers" now plug into the ODBII port instead, because that usually has both direct and switched lines to the car battery.
I believe the small Genasun controllers have diode(s) in them, to prevent a battery from backpowering them and going dead when the panels
have no output. That should be checked with Genasun for the specific model.
Personally I'd stick big alligator clamps on the output cables
, and just hook the Genasun up directly to whichever battery bank I wanted to charge, eliminating all the potential "What if"s that come with whatever previous owners might have done to the wiring