I've done extensive voltage testing, the results are all on that forum link I posted in the previous page of this thread. Here is some of the information:

1. Sound system turned completely down:

14.7v at the battery, fuse box under the hood, rear fuse box, and capacitor.

2. Sound system high volume:

Voltage drops to around 12v and stays around there. It recovers to a higher number when the there is a break in the bass but comes back down when the bass starts hitting again.

With a second battery set up replacing the capacitor, similar results as #2 above.

I was told that when the truck is running, it is the alternator supplying the power anyways and a second battery is important to avoid draining the main battery but more important when listening with the engine off. If I even install the second battery with a split charger, my alternator would still be working to keep that battery charged wouldn't it? its 2400W in addition to the factory upgraded 1200W sound system. The battery wouldn't be able to keep charge long enough without the alternator kicking in for the 2400W total amplifies I figure.

When I replaced the battery about 2 years ago, I did only because of this problem. My original battery tested fine I was told but I wanted to put in a fresh one to rule it out completely. If I plug my battery into a charger after a drive, it shows about 60% charge and goes up from there. Feels like the alternator can't supply the sound and charge the battery enough at the same time. A second battery would only be that much more taxing on the alternator wouldn't it?

At a theoretical 1200w @12.6v you are consuming 95amps
I say theoretical because nearly all music systems stated power output is complete BS, Peak power Vs RMS that is.
Peak music power 1200w = 95amp RMS conversion .707 1200w = 67amp either way its a huge amount of power plus your charge system is trying to run the Full Fat as well.
Obviously it all needs to be in tip top shape

And remember that’s 1200W x 2! There’s two 1200W amplifiers..

I think I might order that super capacitor and see what happens, clearing up a few questions with the seller first.. at least it can be taken out and reused somewhere else if need be unlike the alternator.

Going to the dealership in about a week and a half for some suspension related stuff, going to talk to the service manager there about this as well and see what he thinks..

Is your suggestion alternator too given the situation?

Its a very simple test checking the alternator, if you are going in for other service work definitely get the battery/charge system tested. Its a 5 minute job literally.

Good idea, I will have them test the alternator and battery when I go!