Battery replacement time

I have Costco batteries. They are the low-end Interstate batteries. The 6V GC2 batteries from Costco have a 1 year warranty. I have 2 pairs of them on my Toy Hauler. They are over 4 years old. I have four 100 watt solar panels. When dry camping (which is always) I start the morning off with the Onan generator for the coffee maker, this gives the initial charge to the batteries after running the heater all night long. The solar does the final charge during the day. The older lead acid batteries get the more water they require so you have to check them more often. Monthly I add water. I installed a watering system on the batteries as they are hard to get to. The watering system replaces the caps with a cap with a float on it. You hook tubes between the caps of the batteries and then there is a single hookup when you want to add water to all the batteries.