It sounds like the original 120 volt power came from the #12 two conductor with ground cable you found. You can just put a 120 volt outlet in a surface mounted box on that wire and plug your new converter/charger in.
The #10 wires you see most likely came from the battery and converter/charger to the distribution panel. The battery and converter/charger (original Univolt) would have been connected in parallel, probably with 50 amp fuses in the lines going to the distribution panel.
Most converter/chargers can be used without a battery, but check the information on your Powermax. I don't know what they say about it. But, if allowed by the manufacturer, you can most likely just connect the Powermax output to the red (+) and white (-) lines you have found and you will power your 12 volt
I think I would test the red and white lines first, prior to hooking up the Powermax by connecting them temporarly to a charged 12 volt
battery (put at least a 30 amp fuse in the + line) and see if the interior 12 volt
things such as lights and fans work. If they do, then you can substitute the Powermax for the battery, again assuming that is allowed by the Powermax people. At a later time you can put a battery in the system.
That is my best shot at your problem.