I know very little about electricity!  I checked the archives  and I found this article, but it did not exactly answer my question:
http://gerry.vanagon.com/files/motorhome/Motorhome-Alternator-Charging-Systems.txt

I have 2, 6 volt deep cycle batteries hooked up and I have about 200-ish amps (I think) in the aux batteries.  This is used to run my Waeco fridge, which takes about 7 amps when it runs, but only runs about every 10 minutes for 2 minutes-ish.

Usually I only park for a day or 2 max, so  all is OK with the aux battery, but  this last trip I stayed parked for 4 or 5 days in a row, and the fridge's 'low battery' light came on.

 My question is:  APPROXIMATELY what is the charge rate IN AMPS for a nice good working system.  I say good system as I have a new Zetec engine with a 100+ amp alternator and the wires are all new and the batteries are relatively new.

I use a SUREPOWER 1315 isolator for the 2 battery sets.

When the battery got low, I took to idling the van for about 15 minutes in the morning, and again 15 minutes in the evening, to help the battery a bit and this seemed to help.  Is this folly?

Thanks   Malcolm