sounds like the impedance was too low with the batteries paralleled. you were "boiling" off the fluid.
To charge them, keep them in series. use a quality charger, such as a Progressive Dynamics with the charge wizard. a cheap auto charger will just overcharge them. If you upgrade your converter in your camper to the PD with wizard, it'll take care of the batteries perfectly. You'll still have to check fluid though!
oh, check the fluid often and only add distilled water!
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.