Nothing wrong with AGM batteries (as long as they are true deep cycle batteries) except for their cost !
Charging from your vehicle alternator is not the best, especially if they are down, and I would make sure to get a good quality multi-stage charger that has a setting for AGM.
Als, spend the extra and get a good quality true sine wave inverter. Yes, they cost more, but you will be happier using it with your wide variety of appliances.
Trojan does make 6V and 12V AGM batteries.
If you do add a second battery bank in the bed, you should connect it with Anderson SB120 (or larger) connectors.