In addition to Seth's cautions about sizing and tolerances, and what actually fits the space provided, it also appears that some, earlier in the thread, may have inadvertently wandered into some confusing battery territory. I found that this site cleared up a lot of
my misconceptions and preconceptions when I was researching for that inevitable battery replacement down the road:
Deep Cycle Battery FAQ, and what I possibly should avoid.
I haven't yet replaced my 3-yr old OEM battery, which is an Everstart 24DC. But like most of CI owners, I will have to replace it at some point. The Everstart is generally marketed as a "Marine battery". Meaning that it is a hybrid, with both Cranking and Deep Cycle properties, owing to its plate construction. It is both a quality battery and one that is economical to replace. Although it is by nature a compromise of abilities, it probably won't result in a compromise in performance or convenience for 95% of us CI owners. However, trying to squeeze a few more amp/hours out of a different type of battery (because some of us can't refuse a challenge) has its attendant perils and compromises.
Unless you really wanted the trailer battery to be able to perform as a backup jump-start battery for the TV (which it can), you don't
have to get a Marine battery. A true deep cycle battery (i.e. little to no cranking capacity)
could work for you. Provided that it fits. And you can justify the extra cost. And work with the on-board charge controller. However, I have yet to find any true deep cycle battery which strictly conforms to the Group 24 size class, and has terminals located in the proper orientation, so that it can easily fit in the CI.
Most true deep cycles are made for the solar storage market, where the size of the case is, for the most part, immaterial, since they are stored and connected in large banks. I
think I have found a true deep cycle Group 24 wheelchair battery, but don't want to throw it out there just yet without doing more homework.
A true deep cycle probably won't give you many more amp/hours than the 75-85 that we already enjoy with these marine batteries, whether they're AGM or otherwise; physics has its limits. But their one advantage is that they can be cycled to a lower level without damage. Our marine batteries are limited to about a 50% state of charge (i.e. no less than 12.05V) before we start doing damage to it. This is where your CI-installed Volt/Amp meter are really good at keeping things out of the danger zone. I taped a copy of this chart (see attached photo) under the lid of the right storage compartment as a reminder of how low I can go with my OEM before I have to shut things down, or recharge.
If I treat the OEM well, it may be a while before I have to consider a replacement. By then the battery industry may have evolved even further to producing something with better performance and longevity, and made just for the tiny trailer crowd. Until then, boat owners and big trailer people with more battery space, will own and control the market.