12 Volt system problem

Both batteries are new and fully charged. The light on the control center indicating that the battery is being charged does not light up as it should. I disconnected all battery cables and checked them with shore power connected. The cable shows 4.9 volts. So I have 12.89 volts at the battery and 0 volts at the control center battery cable connection. Then if I start the engine a few seconds later all the lights come on and stay on for about 2 minutes after I shut down the engine. Not a slow fade out. Immediately, like a switch was flipped. I can't trace the battery cable from the lug in the control panel. It goes up into the ceiling. There is no identical cable anywhere on either battery or solenoid. There's a cable to the generator and that's it. The cable to the generator comes of the solenoid on the starter battery. The other solenoid just has a positive cable to the battery, a neg to the battery, and I think only one small wire going across to the starter solenoid. There's also a small wire coming from somewhere that's spliced into the wire between the alternator and the regulator. I've traced it back to the chase used by the control panel wires and it goes up past the control panel into the ceiling.