I've been using an Optima Yellow Top for the past year or so as aux battery
and have always been surprised at how little reserve current it has. This is
based more on intuition than anything else. (It seems like it ought to run
an 800-watt 120 v espresso maker for 5 minutes, but no way...)

But I've just been testing it with a meter and find that the refrigerator
draws it down from a starting voltage of 12.30 to 11.99 in 20 minutes, to
11.84 in 60 minutes. Is that typical?

Also, when the alternator is charging, it shows 13.9 to 14.4 at the
cigarette lighter (vehicle battery) but only a max of 13.7 on the aux
battery. Why wouldn't they be the same?

The standard relay seems to be isolating them as it should.

No, the Optima isn't sealed! There's white powder all over the ground post
and you can see where liquid's been running out, probably eating holes in
the floor.