The "tail/running lights" should be the brown wire on pin position #3 on the trailer umbilical connector. It is the first pin CLOCKWISE from the keyway index. If the lights work when you replace the fuse, then with travel/towing the fuse blows, I'd suspect a pinched wire somewhere along the frame.
First step, at least for me, would be to remove the cover from the umbilical connection box and check to make sure all the connectors are tight and not corroded and that there's no "stray strand of wire" hanging out from under a nut.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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