I have installed two VRLA-AGM 12 volt 90 Ahr batteries. We have not boondocked much, but have used them traveling and when we spent a few days in my neice's back yard. I have never had them go off of the green light on the charge indicator. They are made by Alpha technologies and were primarily developed for a UPS for backup power for computer systems. They are rated for a 10yr service life and have a 5yr full replacement warranty. I have been using them for a year and they have never lost a charge and have been very reliable. I have one in the original battery location and put the other in the basement, inside of one of those plastic battery boxes, with the wires going through the hole that connects the basement to the battery compartment. They are fully sealed so there is no problem with venting.
Fred