Air horns are simple---a compressor fills a tank to pressure, a solenoid valve opens when you activate electricity to it--air flows from the tank then to the horn which has a diaphram which vibrates to make the noise.
Find these components and eliminate them as a problem.
Intermittent probably means electrical---if it were a solenoid not working then it probably would not sound at all. It cannot sound if it does not have air pressure to the tank or if the diaphram is not working---so that seems to eliminate the solenoid and the compressor and the tank and the diaphram.
You should have a dash guage for pressure if it is good then the tank and compressor are fine.
Solenoid valves can be on the horn or somewhere mounted near the tank or on the tank or on the body of the MH. Follow the air lines to find them. They are abvout the size of an old 35mm film can. Two wires and an air hose in and out.
Maybe a bad contact somewhere----a short---a bad ground----or a bad horn button contact----the difficult part is finding it. Unfortunately electrical is the most difficult to diagnose---a good wiring diagram helps---a continuity checker and a multi meter help.
Yes, yes I know my 2003 Dolphin does not have an airhorn---I installed one---you diesel guys can't have all the fun.
Incidently a town horn is not difficult to have nor expensive--and Hadley makes a really nice one---for the future after you solve this dilema.
Good luck to you---if I can help more I wlll assuming this is helpful if not then use your delete.
I do all my own stunts
03 Dolphin LX 6355, Workhorse W22, 8.1 vortec, 04 CR-V, Blue Ox, Brake Pro----Norm, Barb and
Doc(He's a PhD)