Sounds like a direct short on two wires under the bed, probably pinched.
Look for any other blown fuses, especially the 40 AMP Trailer fuse (if it has one), under the hood. If you have the factory trailer package, the brake and parking lights, trailer brakes, and 12v for the trailer are on the 40 AMP trailer fuse. If you have aftermarket trailer wiring, then they are probably tied into your tail light fuse. Check any aftermarket wiring closely. Most of my blown fuses come from a short around the trailer plug. An Ohm meter will measure the resistance through your wires and can check continuity to ground (with the power off to that circuit), which is a short. You can also check the voltage at the tail lights, for your bake lights and parking lights. You should read slightly less than battery voltage, about 0. 1 volts less. If you read a lot less than battery voltage on any single wire, you have a short or bad connection on that wire. If you read a lot less on all the wires, then look for pinched or corroded wires. Remember, a corroded or loose connection will cause increased resistance and can blow a fuse. But it sounds like your main problem is a direct short. I wouldn't go larger than a 20 AMP fuse. Depending on the type of problem or problems you have, a larger fuse could melt some stuff. Good luck!