RJdub
THIS!!
Regardless if you boat is on the trailer or if it is a steep driveway, the brakes should not be locking up.
When your vehicle is put into reverse it should activate the back up solenoid on the trailer.
The power wire from the connector to the solenoid is a blue wire. Check for power to the solenoid. Its normally a fuse, bad ground, blue wires is cut where the tongue swings, and last the solenoid is bad. very few go bad, so check the other items.
You can use the lock out, but if the solenoid is working properly, you shouldn’t need it.
Don