There are several circuits to the trailer connector. Each has its individual circuit protection. I don't think you will find a fuse that covers them all!
I suspect you have something disconnected somewhere. On my 2003 W20 there is a connector on the right frame rail near the steps. From that connector forward is Workhorse. From there back is coach builder. Perhaps that connector has come disconnected. I had one open circuit in the coach builder part of that. I just crawled under the back and traced the wire bundle until I found it.
You might have to do the same.
2003 34' Georgetown on W20 Workhorse Chassis. UltraRV power mods. Front Sumo Springs, Rear P32 Sumo Springs, UltraRV Track Bar.
1998 Jeep Toad.