Does it act that way both on house batteries and when the battery charger has a 120V power source?
I'd start with Hufficon's suggestion and verify the individual house battery voltages and lug/terminal connection quality.
After that, I'd take a hard look at the battery wiring that Camping World installed. Make sure that it's properly wired for series and/or parallel, based on the batteries they installed. A friend of mine had a local RV center replace two 12-volt batteries wired in parallel with two 6-volt batteries using the same parallel wiring...
Next, I'd look at the chassis grounds for the - side of the batteries and the - side of your 12-volt distribution panel.
If that all looks good, I'd check for a blown fuse, bad solenoid switch, or loose/corroded/bad connection in the + line between the battery bank and your 12-volt distribution panel. If there's a break in that line and the Aux switch comes in after that point, it could cause the issues you're seeing.
Electrical gremlins suck, but I'm optimistic on this one. You know that the 12-volt distribution is good with the Aux switch, so it has to be something between your panel and the battery chassis ground. At least you've got it narrowed down that much, already!