This link has been posted numerous times, by several members. Seek and ye shall have knowledge. You will have a better understanding of the differences between the different types of "Deep Cycle" batteries. Not all deep cycle batteries are created equal and a lot of dealers supply a Marine/Starting Deep Cycle battery.
The 12volt Side of Life (Part 1)
Brett,
Your battery is basically new, if you were to add a second battery now, it should match exactly.
Get one as close to the same manufacture date. Use it for one weekend and you have matching batteries.
Personally, I would use the original battery until it is done for. A Group 24 is the smaller of the two common batteries, the larger one is the Group 27.
Like the song says "if I only knew then, what I know now." I would keep the Group 24, get some solar on the roof or even a suitcase panel now. The solar will keep your battery charged while it's in storage. This will allow you to do some boondocking, who knows you might like it. We do.
When the Original battery packs it in, up grade to 2-6V Golf Cart batteries.
I took the long way around through alot of trial and error, 1-12V, generator, 2-12V, 2-6V and finally solar. Much of the trial and error was to adapt our motorcycle touring style to a truck towing a brick.