Gel Cells are the worst option for RVers from what I have found, they are very "Fussy" in terms of charging requirements, Very sensitive to both over voltage and over current (Too fast or overcharge).
AGM's on the other hand, are just about as far from GEL as you can get in this regard, Some brands can suck easily 10 times the charging current that a comparable capacity GEL can take, (heck, 12 times) and they tend to shrug off a minor over charge.
Since both are sealed valve regulated lead acid batteries that can be mounted on end, on side, upside down or in other odd positions..... They often get confused.
OPTIMA, which I have heard called GEL, is really AGM.