How was the electrolyte level in the batteries? If it is low, then they were probably overcharged which would be a charge controller problem. Otherwise, when you say that all 4 batteries are done, are you sure? I brought a battery back from zero volts 3 years ago and it is still working in the Miata. Most smart chargers will indicate a fault and not charge a deeply discharged battery. Some will, but you would have to check the documentation to know if and how to do it. The trick I used for the Miata battery was to put a good battery in parallel with the bad battery and the charger. Then the charger charged the battery. After several hours, the battery voltage came up and I was able to charge without the parallel battery. Or you can use an old school dumb charger.
I have 360 watts of panel and have no problem keeping the batteries charged even through the winter (only loads are the charge controller and the CO/propane monitor). In the summer, I usually leave the compressor refrigerator on and that is a pretty substantial load. I usually keep the display of the Trimetric battery monitor on percent charge so I can peek in and make sure everything is good. If you charge controller has a current monitor, check that to see what going on with your camper in storage configuration.