Being that we had two sets of 12V batteries prior to the pair of 6V batteries we have now, the two 6V batteries perform much MUCH better. Unlike 12V batteries wired in parallel, the 6Vs drain in unison and charge in unison which provide more extended 12V power than two 12Vs.
Our 12V batteries wired properly for power balance, one 12V battery always drained faster than the other. When charging them, one would always get over-charged while the other never gets fully charged. Eventually one battery becomes ill from it all, causing us to replace the batteries prematurely.
Our 12V batteries, year #1 was always good. Year #2 was okay. Year #3 was rocky. Once we tried year #4. As Trump would say, year #4 was a disaster. With our 6V batteries, they are AGM batteries selected for their maintenance-free benefit, we finished year #3 and they performed like year #1. They do so well that I leave them wired up to the rig off season so I have interior lighting. When I power-up, my power panel display shows their voltage around 12.6V which is right where they should be. I do put them on a secondary Black & Decker charger about once every 2 to 3 months to make sure all remains well.
I would definitely exchange the 12V batteries for 6V at Costco. You can get standard lead acid or AGMs there. We bought our 6V AGM Duracell brand at Costco. Tell Costco the truth, you thought the sales guy placed two 6V batteries in your shopping cart. When grabbing a pair off the rack, try your best to pick ones with the same production date code so they are better balanced.
If switching from 12V to 6V batteries, make sure your battery compartment can handle their increased height. 6V batteries are a few inches taller. AGM batteries will require chargers with an AGM setting. So if switching to AGM technology, make sure your RV charger has an AGM setting. I think the AGM setting pumps out a slightly higher voltage.
As far as damage is concerned, some RV appliances may have over-voltage protection built into them. Maybe your generator protected itself by not responding. Items never turned on will surely be fine. The things that don't work at all are easy to identify. It's the things that could die a slow death that are concerning like your slide-out motor. But on the bright side, you learned of the error immediately so I think if it works well, you'll be fine for the long haul.
