Our Ford Dualies had a twenty amp breaker and a seperate fuse under the hood. It came with spare fuses and a puller in a bx under the hood. It didn't contain the twenty amp fuse though.
The repair place had tried to raise the fifth wheel, with a near dead batteries and connected it to the truck. It blew the fuse.
The truck does charge the batteries. Our 05 had two big alternators to handle that and other chores.
I had to go to a truck and trailers place to get a heavy duty connector when it was wired up for towing. Rather I didn't have to but I did. The wimpy little thing a CW and little connectors or crimps that bite through the wire was a no for me. Heavy wire and solder and so on. Twenty amps at a connector that might see moisture going down the road.
:)