First things first, as already mentioned, check open circuit voltage. If you don't get 12.6 volts across the terminals of the batteries stop and start charging.
If you are questioning whether the batteries are sulfated because they don't seem to be "taking a charge" (I used to hate hearing that when I sold batteries...it's not really what happens in most cases...) then I would suggest a 3 minute charge test. Remember though...you have big batteries and big batteries take a long time to charge. For a single automotive battery you are looking at 6 hours at a 10 amp rate to fully charge them from dead. I digress...
Three Minute Charge Test:
1. Fully charge the batteries as best as you can
2. Hook up charger. Charge at a rate of 40 amps (use an inductive meter to verify if at all possible) for three minutes.
3. At the conclusion of the 3 minutes check the voltage across the terminals with the charger still on. If you are reading 15.5 volts or more there is a strong possibility the batteries are sulfated.
When batteries get sulfated it is usually time to replace them. Yes, there are some tricks that can sometimes bring them back, but it's like giving a 3 pack a day Cowboy Killer smoker oxygen...it'll keep 'em alive longer, but the problem still exists.
Do that much and report back. If you find sulfation I can give you some tips on trying to break it up so the bus can make it through winter hopefully.
I also second the notion that you probably have some line drop from connections, bad grounds, etc. You can also test that easily.
1. Hook meter up to positive battery post. You may need a long jumper because....
2. Touch the other lead to the HOT side of what you're trying to test. In this case I would imagine it is that fuse.
If you read more than about .75 volts on the meter (remember...this is a parallel circuit now) I would deduce that you have an issue on the feed side.
3. Hook meter up to negative battery post.
4. Touch ground side of the item you are testing.
You will probably read less than the insulated side because ground is usually a more direct path. Again...if you see a high number it is time to look at what is causing that voltage drop like bad grounds, connections, etc.
Hope that helps some.