Battery question again, mine are shot

[Down South]

I know this has been brought more times than Carter has liver pills but my batteries are not holding charge. Not even with a maintainer hooked up they start dropping below the beloved 50.9V pack voltage. The batteries are 5 yrs old.
My cart, not the one in my avatar is a 2015 rebuild. It has the 6-8V batteries
I purchased the cart used and I could tell that battery maintenance was not a priority from the previous owner and I figured batteries had already suffered before I started babying them.

I saw some at Costco a few weeks ago. Interstate, GC8, RC 110, ah@20hr 160.
I don't remember the price but it wasn't bad. Are these good batteries or should I try to find a Trojan dealer or other?

Anyway, gotta shuck out some bucks for new batteries. Not looking to replace one battery. I will change the pack.

I've got some research to do but I'd appreciate recommendations. I'll add that to my research.