Wet Cell: you have to clean and water: Long life if properly cared for. The most popular is the GC-2 Golf Car battery (Six volt you use two in series to make one BIG 12 volt battery) and these are ideal for RV use.
GEL: NOT recommended, in fact finding GEL cells in the 100 amp hour and up size is kind of hard to do. They used to be popular back in the 70s in smaller sizes but not so much today... They have absolutly NO advantage over AGM and a whole lot of disadvantages. They are very picky eaters (If you charge them too fast you replace them tomorrow) I give them my very strongest negative recommendation.
AGM: All the advantages of GEL (low maintenance, no battery is 100% Maintenance Free) can be mounted in any position (No "This side up). Some makes (LIFELINE) can be recharged very very fast. However much higher cost than flooded wet cells (Roughly the same as Gel).
I have used small Gel's and was not all that happy with their life, even though they had custom made chargers designed to baby them.
About a year after I got the RV I bought a pair of AGM's put them on their own loop (inverter took care of them) and ... They were the first batteries I replaced.
I still have the factory Six Volt Flooded wet cells in place, still going strong. Rig is 8 years old.
