Not as good as 2 batteries, but:
We had only one battery in our 1974, and when we changed the refrigerator, this paused a problem, as the control panel on these "new" models was something unexpected. This is the place where you do not want to run out of juice. So, our fridge now has it's own battery.
To avoid wiring it in the system, we've put it at the bottom of the pantry, and use a Sears battery minder. We have been using that for over 2 years.
There is close to one inch venting at the bottom of the doors, and we make a point to leave the door ajar when we are on shore power.
Maybe a similar set-up can help you.