All evidence points to a bad connection or bad cell in your batteries. When you get home, charge batteries, then turn off everything for an hour or two to allow any surface charge to get absorbed. Then take voltage readings at each battery (with others not connected). You should read 12.7v in a 12v battery. If you do see the voltage, then each battery should be load tested (available at any good battery seller) to see if they need replacing. You might start by cleaning all connections, including ground to chassis connections.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI