Depending on the era and quality level of the rig, some came with inverter/chargers that are little more than a transformer and rectifier...
Newer and/or upper end stuff tends to have smart converters that will go into a float mode to avoid overcharging.
Considering one battery was mostly dry, I wonder if a cell failed in one... which will discharge the other in a parallel system...
Dodge Ram dually laramie 4x4 w/jake, B&W, Brakesmart.
Kit Patio Hauler 394F toyhauler 5th wheel
Camo 680 Rincon, Green 500 Foreman, Blk twincam Roadking