For whatever it's worth, I took a poster's suggestion and bought a 4 amp
Battery Minder for $100, which the Battery Minder folks say might solve
our problem. (What do you think they would say?) These are deep cycle
batteries that were only taken down to 0% only a couple times by
refrigeration and fans in hot weather and charged right back up again
with the engine. I tested them for voltage after a full charge and they
appear to hold it well.

If this doesn't improve their capacity, at least I will have the system
for my next set. For those who care, the 4 amp charger will handle two
different types of batteries (two Group 27Bs for house and one Group 24
starting, hooked up to a combiner) and will supposedly charge and
maintain the set and can be plugged in all the time to keep them happily
de-sulfated, even all winter. I don't take my batteries out of the boat,
ever. They seem to like it just fine, as long as they are charged.

I'll find out soon whether it brings them back, as I'm trailering it
from COLD Minnesota to WARM Florida in two weeks.

Capt. Jeff