The AGM batteries we had in our motorhome so many years were specified to be floated at 13.5 volts to 13.8 volts .... as are the new AGM batteries that recently replaced them.
My AGM batteries get charged and floated most of the time by the same voltage ... 13.6 volts from the motorhome's stock single voltage converter. It just takes longer to charge them at 13.6 volts than it would using 14.X volts. I do charge them sometimes from the engine alternator when traveling between campsites, and they'll get fully charged from the alternator in 4 to 5 hours of driving, depending upon how discharged they were.
The main reason to use higher charging voltages is to speed up the charging process. I see a lot of discussion in these forums regarding possible reasons as to why using higher voltages (than 13.6 volts) is necessary for charging in order to get the longest life from AGM batteries, but I'm not convinced of any chemistry reason for this in everything I've read. If AGM batteries get fully charged at 13.6 volts, then it seems to me that they're as fully charged as they would be had one used 14.X volts for less time.
I wired an ammeter permanently into our RV's AGM battery bank circuit so I can tell at all times how much positive current is flowing into them (being charged) or how much negative current is flowing out of them (being drained). When that meter reads "zero" after so many hours of charging - whether at 13.6 volts or higher volts - then I assume that they're fully charged.
What is very important for AGM (and wet cell batteries) is that you get them back to a full charge after each use. Don't let them sit around too many days or weeks in a partially charged state ... there is a chemistry reason as to why this is hard on them.
