Not all AGM batteries are finicky, and many are designed for true deep cycle use - in both 6 volt versions and 12 volt versions.
Just pick an AGM battery that requires the same float voltage as your converter puts out so that the converter doesn't ruin your AGM battery over time - then use the converter to float the battery when in storage and use the converter, or solar, to charge the AGM when camping (converters work for charging when camping because AGM batteries charge faster on converters than wet batteries do). If you need to charge the AGM battery faster when camping, use a fancy multi-stage charger with hookups or with a generator ... or if you're using a motorhome, drive a few hours to the next campsite with your main engine's alternator charging the AGM quickly.
Pretty simple, really.