Just some general comments....go here to get a better understanding of batteries and their characteristics:
http://www.windsun.com/Batteries/Bat...ting%20battery
It is my opinion that each type of battery has its strengths and weaknesses. I think the factors that decide which battery to use depend on how the converter set up the coach. Space available, temperatures, ventilation, and battery connections all are considerations.
On a coach in which all the batteries are in parallel the owner has some flexibility in deciding how many batteries and what size and type. If the coach is a 24 volt coach with the batteries in series and parallel so 12 and 24 volts is available swapping batteries out for a different quantity of a different size may not work without revamping the battery wiring.
If a coach has AGM, switching to a gel may not work with out insuring the location is relatively cool, and without changine the charging voltages. On the other hand, switching from a gel to AGM is OK and less likely to result in failed batteries even if the charging voltages were not changed.
Do not expect the battery salesman to have a clue about your batteries. You need to make sure you understand how your inverters or chargers are set, whether the location is OK for the type battery you want to buy, and how you use your coach determines to an extent what batteries will work best for you. The salesman will sell you anything you say you want.