The only time you will series a battery is if you use two 6 volt batteries run together to achieve 12 volts. Never series a 12 volt battery in a 12V boat application unless it is for a trolling motor.
Weight is the great equalizer for specifications that may or may not be accurate. Its highly unlikely that you will get more than 80 amp/hours from a 50 lb Group 24, 95 amp/hours from a 60 lb Group 27 or 110 amp/hours from a 70 lb Group 31. A heavier deep cycle battery will certainly have more amp/hours and a lighter battery will have less.
For consumer grade deep cycle battery longevity you would ideally like to keep your depletions limited to 50 percent or 12 volts.
For worse case, at 50 percent depletion, estimate around 50 amp/hours of battery capacity for every 1000 watts of Class AB amplifiers driven at full power for one hour. Class G/H is more efficient yielding more time. Class D is 60 percent more efficient yielding more time. Your system efficiency (including sub enclosure, speaker efficiency, amplifier efficiency), accuracy of power specs, age and condition of batteries, starting charge of batteries, regulated or unregulated amplifiers, how loud you play it, quality of downloads, system tuning, equalization, music type, etc. can have a profound impact on the play time at rest. Music is transient in nature so it's going to be a very different calculation than DC bulbs for example. Music can represent a very wide dynamic range at a moderate volume to a very compressed dynamic range as you approach full volume so this can also be a major multiplier of play time. Compression consumes much more power.
For an AC shore battery charger you want a smart multi-stage, minimum 2-bank (depending on the amperage capacity required you may be forced to get a 3-bank charger even though you will only use two) with a total amperage capacity between 10 and 13 percent of the collective amp/hours of your batteries. This is required for proper desulphation when you are seriously deep cycling batteries. Tenders/minders/trickle chargers are for those applications when the vehicle is placed into storage with the batteries fully charged...but not good for the above usage.