FIGURE Trailer Lighting Provisions(c)
Looking at the schematic I would say it has to be in the trailer, check that circuit for the park lamps and make sure your park lamps on the trailer are not drawing more than 10a if they are 8a or higher I would try A 15a fuse. If ya can try installing A circuit breaker where the fuse goes with an amp gauge connected to it(not sure where to buy A tool like this I built my own) and then you can monitor the circuit to see if it spikes.