12 volt rv battery vs 6 volt gc batteries

I never had problems til I got Costco 6v's. I got 8 yrs out of T105s and 8 yrs out of Napa HD 6v's, both being maintained by a charge wizard. I just took out my Costcos this morning and checked with a hydrometer... exactly 2 yrs old and they are junk! They wouldn't even run the furnace one cycle this weekend. They were maintained by a solar smart charger. I might go back to plugging in so the charge wizard keeps them up.
Even when the costcos were new they only had 2/3 the capacity of the other two brands. I think the problem is partly Costco and partly the solar charger not being able to do the job that the charge wizard/converter can do. Craig

Same problems can exist whether it is two 6 volters or two 12 volters. Bad, poor connections, or incorrectly connected.
What is the problem or issue that you have?

Without knowing what kind of trouble, it's hard to say. I have a million threads on the 12v vs 6v subject.