I don't think you have a quantity of batteries problem. Adding another likely won't help. If you can notice a change in battery behavior at low RPM's I'm betting you have a problem with your alternator(s). It's not normal to see a change at idle/no-wake. I'd suggest you get those checked ASAP before you wreck your relatively new batteries. With four properly charged and properly cared for batteries you should be able to go for 2+ days with two fridges, radio, head, etc. running. With all of the above said I suppose it would be worth asking what size batteries you put in 2 years ago? Are the group 27's at least?