What Berzerker said, plus a 12 or 24v maintenance charger, either plug in or solar to provide the make-up/maintenance energy.
Battery balance in parallel batteries, since they are tied together, is purely a factor of internal resistance and battery chemistry.
Basically one battery will be electrically larger or smaller than the other. They will charge and discharge at different rates which will increase the difference between them over time. Since they are always connected, a larger battery will constantly feed the smaller battery. Since most people dont take the time to charge, load test and match batteries they are tying together in parallel, and probably dont have a truckload of batteries to choose from to pair a matched set, this is yet another good reason to drop to just 2 batteries in series... that is afterall what this is, puttinf two batts in parallel to make a bigger 12v batt. The truck sure doesnt need 4. If you absolutly are convinced you gotta have more battery, get 2 larger batteries in series... you are still stressing the hell.out of the alt though if they get discharged.
Batteries in series can also experience imbalance issues for the same reasons, BUT ONLY WHILE CHARGING AND DISCHARGING. Like in parallel, mismatched batteries will overcharge one and undercharge the other which will shorten their life. The good thing about series batts is that those imbalances can be compensated for using a balancer. This will extend their life. By keeping them at nearly the same float level, and help compensate for charge imbalances/electrical size differences.
With a balancer in place, it does not matter where you apply power, 12v on 0-12, or 12-24 or 24v across both series batteries. The balancer will try and keep the two 12v batteries within .1-.2v of each other.
I had a small solar 12v float charger hooked to my 0-12 battery because that was where the vampire load is on the A0 trucks. The balancer kept the 12-24 battery floated within .1-.2v of the 0-12 battery voltage.
The only real issue you can run into is if you apply a large 12v load with the balancer connected. It will attempt to compensate for the imbalance this causes, by shifting 12-24 energy to that loaded 0-12 battery.
It is simple enough to wire the balancer and charger to a multi pin plug so all the battery connections are hard wired and all you do is plug in the charger+balancer when you park the truck...
