You might try hooking 12 volts to the horn directly to see if it is working. If it is, about the only thing left is the horn switch, fuse, and wiring in between. I don't think there is a relay involved. A good ground is important also.
1998 Min Winnie, 2000 Winbago Journey, 2015 ACE 29.3
2016 Thor Miramar 34.2