Well the "best" 6v batteries are from Trojan. But Costco ones have a very good reputation. Filling them with distilled water twice a year is pretty important, as is how they are used/maintained.
Do you make sure they are fully charged after every trip? Do you draw them down lower than 11.8v during your trips? What do you use to charge them?
They don't need to be hooked to a maintainer all winter, but they should at least be fully charged before storage, one in the middle, and again before you use them for the season. Most people find it simpler to just leave them hooked up to the maintainer.
Edit, just had a thought, do you have solar? It's easy for solar to boil or overcharge them. They'll use more water on a solar system.