How to tell

sorry to take so long, but I knew I'd be getting the truck out soon, so I waited. Truck not running, but hooked up and shore power unhooked, batts read 13.0 volts. Start truck and read again 12.99 volts. Found the relay and fuse on the truck labeled aux battery, fuse is good. I really don't know where to check. I may have to go to the local trailer shop and see what they say. Thanks for the help, by the way...

Put your volt meter onto the 5er battery, fully charged should be at least 12.6 volts, plug in your 7 way and start the truck engine, take another volt reading at he 5er battery. If it is charging the reading should be at least 13.6 volts. If the reading does not change from your first reading then the truck alternator is not charging. Then the trouble shooting begins. Report back your findings and I am sure you will get help in finding out why?

You can also check at the plug itself if to make sure you're getting voltage to the back of the truck. Pin 1 = Ground, Pin 4 = 12V.

Easy, but you need a voltmeter. Fords have a relay so they only power the 7-pin when engine (or ignition?) is on. So plug in the 7-pin with truck off. See trailer battery voltage at 12.x. Now turn on truck. Trailer battery voltage should go to near same as truck voltage-- 14ish.