Suburban won't start unless jump started. Why?

What is your question?

I have a 1999 K1500 Suburban 5.7L 4WD. I mainly use it to tow my boat to and from the boat launch, less than a mile round trip. Since the truck sits for a while between uses, I put a battery disconnect on the negative battery terminal so that it doesn't drain between uses. The other day, it cranked once slowly, then the starter just clicked. It sounded like it was a typical low battery since it tried to crank so I jump started it and all was fine. I let it run for 20+ min while I got the boat hooked up so the battery could charge, then went to the lake. Put the boat in the water, parked the truck and disconnected the battery just in case there was a drain. Went to take the boat out of the water, reconnected the battery, and again it cranked slowly and wouldn't start. I put a new battery in it since and it does the same thing. Could it be a bad battery ground? When disconnected, the battery reads 12v but connected it drops to almost 7v. Any ideas?

How long has your 1999 Chevrolet K1500 Suburban had this problem?

A few weeks