Welcome, I am Chris (aka-Moose)
If you unhook the trailer, and use a test light on the connector on the truck with the park lamps on, does the fuse still blow? If not then it should be the trailers to blame. Another test is to use a test light like before, and a digital multimeter in place of the fuse to see how many amps the circuit and test light draws. Then use your test light direct on the battery and see how many amps the light draws. The readings should be the same. If they are then the wiring on the truck is fine.
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