The coach batteries in my motorhome are starting to show their age (6 years old) (accelerated because I once accidentally drained them to dead-zero) and I need a couple replacements. The current ones ...
You are much better off getting 2 6V golf cart batteries. Both Costco and Sams carry them at a very good price.
AGM have their place which is where they might wind up on a 45° angle ! Yes, you need to water flooded cell batteries. Yes you should clean the outside at least once a year. Doing that maintenance will save you a lot of money.
It is also difficult to find true deep cycle AGM batteries.