My trailer is a 5 pin. and i have the 7 pin plug with a 5 pin connection and my trailer will still lock up if i try and back up a hill....

Check the fuses on the truck. Often they are designed as such that the factory config does not power the electric brakes or reverse 12V+ without inserting a fuse.

Check to make sure you have 12V+ at the reverse pin on the truck.
Check to make sure you have 12V+ traveling all the way to the solenoid.
Check to make sure the solenoid is working. (You should hear & feel a "click" if you have your ear near it, and hand on it when someone else puts the truck in reverse.)

If you have power all the way to the solenoid, replace the solenoid. They do go bad. I always used to carry one with me.

You mght have the standard reversing solenoid. It won't bleed off the pressure in the lines. It just closes the line off. A real problem if the trailer is pushing against the truck before you put it in reverse. Such as backing up an incline or being stopped and not pulling forward a hair before going into reverse.The deluxe solenoid bleeds off all the pressure. A must fro trailers with disc brakes.

Good info will check all of these things, next time i hook up to it.