Six years is about the normal life expectancy anyway. For $80 Costco has brand new marine/rv "deep cycle" batteries. Not a true deep cycle but undoubtedly better than what you have even if you get it to hold a charge.
I am getting one more battery box and getting 2 GC2 6 volt batteries, also at costco for about the same money. They are true deep cycles and are made to be discharged further without damage. With new cables and boxes it will be about $225 for way more than double capacity. Should allow me to run anything I need overnight without worry that I will wake up cold. :)