well, I typed up a very detailed response...got sidetracked for a few minutes and the system logged me out and lost the whole thing.
these batteries are harmed if discharged too deeply, and/or after a lot of discharge cycles. take them down to deep and they will never again hold a full capacity charge. I've had car batteries that would not take a charge again after two cycles down to near 0 volts
Those starting batteries can't handle much at all.
The "marine/RV Deep Cycle" batteries can handle more, but not truly deep. general rule of thumb I most often see is 50%...never take it below 50% charge... I think the actual limit is less than this, but that's what most folks tend to preach.
Most folks say 50% for golf cart batteries too. people call them "real deep cycle batteries". the are made to handle it more. The trojans in my golf cart came with a manual that said something like 30%...and I'm sure there's a safety factor in that too... but a lot of folks say to use 50% for them too
Point is, don't discharge them too much or put them to bed empty or they won't hold as much moving forward.
so I like to install mechanical switches on all my batteries to physically disconnect them when in storage... you never know if a radio is using a bit of energy to maintain memory...or a CO monitor.... pulling it down while in storage
The other thing is self discharge. These batteries (lead acid) will discharge even when nothing is attached, nothing being used.... it's more complicated than a fixed number, but I've seen 3% to 5% per month and I sorta feel like this is the right ballpark in my experience. So a battery sitting on the bench in storage with no wires attached and so no load, will be 3% to %5 down after a month. Store it a long time and it will be dead.
this is when you need a battery maintainer. If you are storing for longish periods of time.
I try to be mindful to put my RV to bed with as full a charge as I can... and it's usually stored no more than about 3 months before we use it, so I have disconnect switches on the batteries and that's the first thing I do when parking...disconnect them...but I don't need a maintainer really. I don't use my boat very often at all, and so I have one on it. like this
...but there are lots of sizes and styles
This all was hard learned by killing and replacing many batteries on my boat and campers over the years.... now my batteries last a long time typically.