I agree with Tuna. GEL is the WORST choice
That leave
AGM
Maintenance Free
Flooded wet cell
Flooded wet cell is lowest in cost but highest in labor, you have to water them, clean them and otherwise maintain them fairly often. That said the most common Flooded wet cell RV battery is the very popular GC-2 Golf car battery. The GC-2 is used in Golf Cars (GC) all over the place, Golf courses buy 'em in bulk and thus they are less expensive (per unit) to make.. They are also DEEP CYCLE batteries. True deep cycle.
Maintenance free, is a wet cell as well so must be mounted "THIS SIDE UP" as it were. (Caps on top) however though it is becomming easier to find 12 volt DEEP CYCLE both in Flooded and maintenance free.. most of the Maintenance free sold for RV use are Marine/deep cycle, this is not a deep cycle battery by any stretch of the imagination. They are a bit more expensive.
AGM: Are the most expensive, the only real advantage over Maintenance Free. is the lack of a "This side up" AGM are a bit more rugged, and LIFELINE claims a faster bulk charge than any other battery maker.. But in truth, the faster BULK won't save you much charging time.
In my expierence (MK/Deka) They do not last as long as Flooded.