There are two fuses that each contribute to the trailer plug, one is in the engine compartment fuse panel and the other in the passenger compartment fuse panel. I have attached illustrations below showing which fuses to check
I just double checked my information and you are correct, I mistakenly retrieved info on an E-150, give me a few moments to get the correct info for you
Ok, I apologize for that, please disregard the previous information, the trailer fuses are all in the engine compartment fuse box, I have attached a new illustration below indicating which ones to check, Were you able to get the lid off yet by depressing the tab on the side of the box?
The bulbs in the trailer use different filaments for the park lamp than the turn and stop lamps, are you sure the bulbs are good? Have you checked for power at the plug with a test lamp and the park lamps on?