Crazbs,
You have most of it right and Pianotuna has a lot right too.
If the disconnected batteries are good, they should only discharge about 1% of their capacity PER MONTH in the cold. If you are worried about that, go find a couple of solar battery keepers that they use in car storage lots. These are cheap and they are about 5 watt and that is plenty to offset the leakage discharge.
I (as an engine professional for multiple decades) am convinced that the "Exercise The Generator" is put up by the people that sell parts for them. I guess it may be good if you can't follow the instructions to drain or at least limit the left over fuel in the carburetor. I have several 40+yo Onans and a Honda near that same age. None get regular exercise and all will start right now if I put fuel in them. (Except for the waiting for a minor repair.) So, find out how to drain the carburetor or at least run it out (draining is better).
Matt