For those smarter than me - - -

To quickly verify the ground connection, use one cable of a set of jumper cables to connect your truck to the trailer. Remember to get a good connection - you might have to wiggle the clamps.
Hope this helps.

Agree on the ground, it can cause all kinds of problems. I at one time had a Buick Skylark and the windshield wipers would not go off. It turned out that it was caused by a bad ground on the wiper motor. JMO

I too will support the bad ground.. Let me tell you one......

Lights on Motor home work well till I plug in the towed, then suggenly with head lights on the turn signal does not flash.

Solution,,, Improve the ground connection at the rear of the motor home for the lighting connector.. yup, that was it .

Could be a corroded lamp socket. Check socket and base of lamp for corrosion between the contacts.

Along with the above replies, or a bad ground wire on trailer, you may be a heavier duty flasher (turn signal) for the truck. If the flasher is faster with dimmer light than the truck alone, this may be the case.

Likely a ground problem somewhere. If you have not, hook up and drive around the block a few times. That sometimes solves a grounding problem by cleaning the cup on the ball. A direct ground wire from trailer frame to vehichle frame would also solve that problem. Trailer lights seems to develope thier own "ailments" from time to time and it usually involves a grounding situation of some sort. Good luck..

If trailer tail light checks out, go to the right tail light signal at the tow/trailer plug to confirm that connection.

If you have not done it, make sure the right signal on the tow is strong with the headlights on. Headlights are drawing enough power on the circuit that any weakness in the right turn signal becomes more visible.

Bad ground at the right trailer turn signal, pull the bulb and check all conenctions.