Speaking of batteries...

I have two deep cycle Exides that are almost 7 yrs old. They are holding fine. I run furnace all night and have all incandescent bulbs inside the camper.

I go the least expensive deep cycle batteries with the best non prorated warrantee.

Same camper, but with new batteries!
Batteries seemed fine when we headed out to Alaska at the end of May. Worked fine until mid July when they just up and wouldn't hold a charge.
I think they didn't like powering the inverter the 2 or 3 times we used it.
Replaced with a pair of Group 27 from Auto Zone in Fairbanks.
Had no concerns after that about having enough power. We were driving most days so batteries were charged back up, and also got charged by campsite power when we had it.
Auto zone has a tester that will tell how they are holding charge, both mine were not doing well. Charge up the batteries and take them in for testing. Only problem is you need to take them into the store. No fun wrestling those weights out and back in.
Brian

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.