I'm not the right guy to answer this.....I have fought batteries and chargers for a while and finally got my system working (took new batteries, since the charger was overcharging the old ones and I wasn't smart enough to realize it...it was pushing them to over 14 volts and was using over 100 amps (at 12 volts) to charge them. Once I got a new circuit board for my Magnum (just call their tech support and talk to them...they are really helpful and will walk you through the tests and then make recommendations) all is wonderful with my new batteries.
So, I have had to take them out and put in new ones...not too hard. Just take photos of the cables....label them WELL, and then draw a diagram too. They should be two six volt batteries hooked up in series to give you 12 volts. and then another pair also hooked up in series, and then the two pairs of batteries are hooked in parallel to get you double the capacity.
Sounds like you are pretty sure they are getting the correct charging voltage. I would start by pulling them out, and having them load tested....since two boil and two do not...you need to know why they aren't responding the same.
Again, call Magnum first and talk to their tech support. Good luck.
'07 Holiday Rambler Endeavor PRQ ("Elvis"), and our 2012 CRV toad