The Mastervolt is a good unit. I believe the first thing to do is to try to find out why it's not working properly. 13.2 volts is the float voltage. Have you tried the Mastervolt on a deeply discharged battery bank? Also, do you have other charging sources attached? Wind generator? Solar panels? If so, they may be "fooling" the Mastervolt into believing the battery bank is fully charged.
The Iotas are very wonderful chargers, in my experience. While they are not "marine approved" and do not have temp sensing, they are very well constructed and do extremely well in marine applications. Many others have found this, as I have (I have two of them, and have installed several others). They are economical, virtually RFI-free, tolerant of voltage fluctuations in the AC supply and, with the IQ4 option, provide true multi-stage charging. They also have an automatic "equalization" cycle if you haven't charged in over a week. It's not a true equalization (which would require 15.5VDC or more on a 12VDC battery bank) but it's helpful. Further, they seem to maintain battery capacity better than even high end chargers I have used (including a top-of-the-line Victron MultiPlus).
AGM charging voltages are VERY close to flooded battery charging voltages. I wouldn't hesitate to use an Iota charger on an AGM battery bank. Because of the PWM (pulse width modulation) technology employed, Iota claims they don't need temp compensation since they can't overcharge/overheat a battery. Maybe, maybe not (there's some discussion re: the type of pulsing employed). But, it's also true that temperature sensors can be off -- in both directions -- and can cause problems. Maybe that could also account for your problem, if the Mastervolt sensor is over-reading battery voltage.