I carry one of these 7-way Trailer connector tester units in my RV Tool Box. It's always nice to eliminate the truck side when finding problems like this...
I got mine from ETRAILER...

I had to replace my trailer cord and plug one time and was an easy job as the leads were all color coded... Color for Color in the Electrical junction Box just under the frame... Mine used the large yellow twist on connectors...
By the way I had to install a small relay and fuse in the truck fuse panel that was in an envelope in my 2010 Ford Truck glove box when I purchased it new. No markings on the envelope. This allowed the truck to provide 12V CHARGING for the trailer only when the Truck Ignition Key was 'ON'...
Roy Ken