Gel Cells have had their day. They are too picky about charging and use compared to other options.
Lew is a bit optimistic, I think, about the life expectancy for AGM's. Az Wind Sun
, for example cites 4 - 7 years. (it is an interesting chart, wish I could see the raw data. it tends to refute several common battery myths)
AGM batteries are nice if you can't store them properly or have a need for large discharge rates (e.g. inverters) if cost isn't an issue. They really don't gain you that much for typical RV solar service from anything I can see or the experience I have had with them.
The biggest battery problem I see with RV solar systems is either with underpowered panels that have trouble keeping the batteries charged or over potent panels that tend to float the batteries. Lead acid batteries need appropriate exercise and full charges to maintain good health.