AGM batteries, overcharging on long drives

This is a legitimate concern and part of the answer is the battery manufactures own literature i.e. when I bought Lifeline 150 AH AGM battery I checked the specifications put out by Concord/Lifeline. An AGM starting battery is a different animal than a deep cycle AGM, as much as conventional starting battery is to a true deep cycle.
One alternative is to use a three stage regulator i.e. Balmar http://www.balmar.net/regulators.html