Scoto...let's first get the language straight. There are WET cells and there are AGM's available in deep cycle configurations. Absorbed Glass Mat Batteries are 2-3x the price of wet cells and do not require watering. They also have many other good features that some need and are willing to pay the price for. They do NOT provide longer life..indeed they offer FEWER recharges but often work out lasting longer when compared to wet cells that have been murdered! LOL.
Wet cells come in 2 versions..those you fill with water...and those which contain an internal resevoir and vent system which makes them "maintenance free"....and cost about the same.
THERE IS NO SUCH THING AS A TRUE DEEP CYCLE MAINTENANCE FREE DESIGN.
If you see one advertised...you are looking at a dual use battery and a poor choice for your RV unless you do little or no boondocking.
If you don't need or want the spendiness of AGM's...I suggest true wet cells in a deep cycle design. Then, if you find filling the water to be a pain...you can get something like the pro-fill systems to make it easy.