Strange low idle

So for the past Id say 3 weeks my 03 SSS has been idling kind of low right around 500-550rpms in gear with the truck at full operating temp and about 700-750rpms in park and I cant figure out why. I cleaned my throttle body and MAF and none of those really made a differece. I was told it could also be the fuel pump or fuel filter but Ive also been wondering if its something to do with the battery. The reason I kind of think that is today while trying to see if I could figure out what was going on when I pulled in my drive way and put it in park I noticed that with the windows all the way up when I pulled up on the window switches the headlights would dim and the rpms will drop down a little bit and I can actually hear the engine sound chance when I lift up and let go of the window switches. I kind of noticed the truck idling low a few days after I installed my stero system which is on the small side it consists of a pioneer avh4100dvd radio with a rockford fosgate t-400 am powering a 12in sub and it uses a 4 gauge power wire that goes through the fire wall to the battery with an inlike fuse and fuse holder and I had this exact set up except I had it going to an optima yellow top in my 5.3 Silverado and when it got totalled I pulled it out and put it in the SS and I had no issues at all in my 5.3 with the yellow top. The SS has an interstate battery that looks like it was put in january of 08 so I was thinking maybe the battery is either too small or just worn out and doesnt have enough juice to power everything the right way? I unhooked the power cable for my amp from the battery and it made no difference. Im open to any thoughts anyone might have, im a member in the Silverado SS forums and asked there but I was a member here first and Ive found this place is the best for technical questions! Im hoping to get this figured out ASAP because im going to be driving from CT to FL in a couple weeks and riding dirtbikes for the winter and I dont want to have to worry about it getting worse when Im down there!

My SSS had an idle of 525 in the stock tune. It's had a few changes and no longer idles there but I never had any issues with it.

Just wanted to say I truly doubt it's the battery. If you have a bad (or weak) battery, it will drag the starter upon startup, but should have no effect on the idle speed of the engine while running. Think of a dead battery you have just jumped and are allowing the alt to recharge ... no effect on the engine idle.