Check & see if your trailer lights connector is covered or open. If open, damp conditions can cause a short to blow your fuse.

I had the same situation, but found out it was the 3rd brake light wiring I ran to the tire carrier. The wire rubbed through the coating and kept shorting.

On another occasion it was the parking light connectors. I took the lights off & every time the connector got wet with the lights on, the fuse blew.