Keep in mind the Charge line from the Truck's Alternator is pretty small gauge wire likely 12 gauge or if you are lucky 10 gauge. You will get a pretty slow charge rate from the Truck likely around 6-10 amps or so. The Converter Charger in the camper is likely a 35 to 45 amp charger and as previously mentioned you should see at the very least 13.2 volts at the batteries preferably more if not the Converter Charger is bad or has a blown fuse. Hopefully you do have 2 deep cycle batteries!?!?! If not look at upgrading the battery bank. Remember Randy at: http://www.bestconverter.com/ BoonDocker Converter Charger are great.
GARY