Battery Voltages vs Capacity
Yes you do. But the key is knowing which problem exactly. And after knowing that, you will have the correct answer.
As Jack expalined, conections are usually the main source of problems. Check wether the alternator is providing at least 13.8/13.9 v at the battery terminal when the motor running. You should also check connectors at the battery, looking for any loose bolt or corroded or sulphated terminal, as well in the alternator, thru contact keys, and isolators and fuses in the cables running from alternator to battery bank, if any.
We do assume that electrolyte level is OK, and never happen the level to be below the plates.
However, and sorry for this, according to the symptoms, I guess your batteries are about to die. Probably due either to a low electrolyte level, or undercharging.
If they are less than 18/24 months old, probably they can be equalized and probably more acid be added in a work shop.
Some Ac chargers have a feature by which you can equalize them. This consisting in charging the batteries with 15.5 volts during one or two hours. This will make an acceleration of the chemical process allowing the convertion of the salts adhered to plates. Accumulation of this sulphates is the natural process of living and dying of chemical batteries. Calder''s book ilustrates this from square one.
Even if you buy brand new batteries, I believe one should understand what happened.
Good luck, and keep on trying