Jay, first off, you can add 10 batteries to your truck, but while it is running, if your voltage is dropping to 12V, the batteries are not going to fix that issue. Batteries are really only there to provide a means to start the motor and to provide a limited amount of noise filtering. End of story. The only exception to that is if you need power with the motor off. But, I don't hear you really saying that.
So, really, getting a larger battery would serve your needs with getting more CCA (cold cranking amps). What you really need is a larger alternator. Based on what you are saying you have on the truck and what you are looking at running, I would be looking at something in the 200 to 250 amp range and then doing a wiring upgrade to replace the wiring between the alternator and the starter solenoid. I liken this to trying to fill a pool with a garden hose vice a fire hose. The garden hose works, but you have to stress it tons more than the fire hose. The factory alternator wiring barely handles the 130 amp alternator. Will the wire flow the 200 amps, by all means. But, does the term "toaster wire" mean anything to you? The more current you flow through a given wire, the more voltage it is going to drop. Since power loss (aka, heat) is a function of voltage drop times current, going from 130 amps to 200 amps is going to almost quadruple the heat that the wire has to get rid of. What does this mean? I will put it in 2 simple things: 1) imagine a wire hot enough that you can feel the heat coming off of it, or 2) BBQ'ed Ford in extreme conditions. I am not trying to scare you, but Ford did a lot of research to figure out what is the minimum gauge wire they needed to minimize weight in the truck. Going above and beyond what the factory truck requires is pushing things too far. Hell, in some cases what Ford installed is not enough (headlights is a prime example).
You may find simply upgrading your wiring between the alternator and the starter solenoid to say a 2 gauge wire (from the factory 8 gauge wire) will make a big difference.
The other thing that most people do not realize is that while we may have alternators in our trucks that are rated at 130 amps, at idle, they are limited to about 65-70 amps. The current output is a function of the field strength (ie, how much power the alternator is sucking back in) and the speed that the alternator is turning at (ie, function of engine RPM). There reaches a point where the alternator can not suck in any more power to make a stronger field due to the internals saturating. Hence why the output of the alternator drops off. This is why you are seeing the voltage drop to 12 V. When you are seeing the voltage drop to 12V, you are truely running off of the battery which has a finite life to it. A battery only stores just so much energy. Sure, adding a second, third, fourth battery will extend that time, but you are just putting a bandaid on the real problem. Besides, overloading the alternator is significantly shortening its life. How long do you think your truck would last if you only drove it 2 ways, foot flat on the floor for the gas or foot flat on the floor for the brakes? That is what is happening to the alternator.
In your line of work, you want coolness, get yourself a 250 amp alternator and a set of 2 gauge jumper cables. Imagine being able to jump start a truck like ours with a dead battery just by connecting the jumper cables and having the other person turn the key. No waiting, just turn the key and their truck rolls over. I think in your line of work that would be much more impressive than "hey look, I have 2 batteries".
I am not trying to steer you away from what you want. But, I am a function over look kind of person (as nukie reflects). I make sure the truck will do what you want, not just look cool.
