It's likely a fuse in your truck. Just to make sure, if you have a tester, go back to the electrical plug in on the hitch of your truck and test running lights to ground. If it's not showing voltage, you've at least isolated it to the truck.
Also check simple things like a dirty connection. Sometimes wiggling the connector in the plug can correct the problem.
Also, check to see that your truck's lights aren't on auto on/off or something.
Just a few thoughts. Hope you get it sorted out.
2017 Forest River Surveyor 247BHDS...
...Tugged by a 2016 F250 XLT FX4 SuperCrew, Shortbed 6.2L, 4wd, 3.73...
...Joined with a Husky Centerline TS 800-1200lbs WDH