Flat to hear you actually have 6 volt batteries.... Wired in series!
If they were 12 volt
batteries in series you certainly would have cooked most of your devices!
My suggestion is this....
If your batteries were on fact void of electrolyte, even a little bit, your batteries are forever damaged, and will lead to a very disappointing camping trip. Despite topping them off, and them taking a charge, they are permanently damaged. Buy 2 new batteries and maintain them monthly!
Add grease to the terminal ends, monitor the fluid level, replace your stock converter with a 4 stage model, add a little solar charging ability so when you're not using/ plugged in, you'll still be charging and maintaining passively!
SAMs club has decent pricing on golf cart batteries....
Lifeline, Trojan, rolls are all fine battery mfgrs. Stay clear of Optima. It's just a pretty face.
Battery don't have to be a scary decision, nor does maintenance. Just keep everything greased and charged, and the electrons parade will always be sunny!
Good luck, and google Fluke meter!