Not all 6 volt batteries are the same. Buy Industrial not commercial grade and you get what you pay for. Buy cheap get cheap. I bought Trojan t 125 forklift batteries for my first TT. Sold it at 10 years Batts still going strong. Second TT it was a no brainer, bought another set but after 4 years we decided to trade up. Bough a fifth wheel and took the Trojans off of the Sunnybrook and installed in the fiver then added two more. Four batteries 480 amps. Yes they are $450 for two, but 10+ years of perfect service, is worth it to me as we dry camp more than plug in.
What you really need to understand is that flooded cell batteries if you discharge them more than 50% you shortening their life. After using a VOM to check batteries I installed a Trimetric battery meter. Just got back from a week on the beach. Dry camped and with solar and inverter never started the generator.