Oxford has the right idea. You have to see if you have an electrical path from the alternator to the battery when the van is running. You should measure over 13 volts.
Isn't the Xtra an Xplorer? Those old dogs usually had a diode isolation block located on the firewall right behind the chassis battery and they are notorious for failing over time. And. yes you should check the fusible links. They are usually located in the wiring harness running along the top edge of the front lid. I've had problem with these links where Xplorer patched in wires for some of the RV circuits.
Good luck.
karl