Check the level after charging and sitting for your 8 year old batteries. Likely they are doing just fine, but compare them to new voltage batteries.
Batteries age, just like people do. After a while, they don't last as long. But if they still take a charge, and give it back, it's worth keeping them for even longer. You have a generator and about two hours a day while boondocking will usually recharge most batteries to a good level for the next day. I would just keep on going.. I watch my battery voltage with a cheap meter, and I can tell when the batteries don't get up as high as I would like them, and I can plot their decline while I camp. Knowing that, I make a decision to keep or replace, but never go the cheap way along the way.