I am planning on adding an additional battery and isolator to run accessories and an inverter. Since my truck is factory dual battery under the hood isn't an option.

Who has batteries mounted in locations other than underhood and where do you have them? In the bed isn't an option.

Truck is a 98' K-3500 long bed.

The GM military trucks have a third battery mounted under the bed on the passenger side in front of the rear wheel. Thats on the newer body style but I would bet there is room in the 98.

I don't think I checked there. I was laying in about that spot when I looked at underneath the cab and don't think I checked under the bed at that point.

Its a pretty big space, the width between the fender and the frame is wide then the bottom of the bed is pretty high there so you have a good bit of space.
When I had my S10 there was a body line right at the bottom of the bed. I thought about cutting there and hinging the bottom and boxing it in on both sides then have a storage place on the drivers side and battery box on the passenger side.

Here is a picture of the 3rd battery tray. Try this link and click on parts manuals. 2006 LSSV parts manual supplement. It is a pdf file. Go to section 4-7 to see pics and parts needed. Hope this helps.

http://www.gmfleet.com/government/products/GMDefense.jsp?navSec=products