I just looked at some pictures of a Barth Regency and they all had Hadleys right on top? Look there? Hey nice looking MH, so congratulations.
Air horns are simple but you need to find it and its source of air pressure. When you do the key elements are the solenoid and the pressure tank and the horn itself. You may have a horn that is one unit like a WOLO so no tank. If you do not have them on the roof.
The main reason for air horn failures are a stuck solenoid or no electricity to the solenoid, a gunked up horn which needs cleaning or/and a new diaphragm, or no air pressure from the source. If the source is a tank then you may have a leak or a break in a line
Some air horns are all one piece, with a compressor activated by electricity which powers the horn. If that is the type you have and the problem is electricity to it the the unit may be fine. If not then you can find replacements on line for not a lot of money.
I suspect the problem is electrical or a stuck solenoid. When you find it have someone hit the button while you see if you have electricity going to the solenoid, if so then move to air pressure.
A solenoid looks like an old 35mm film canister. Sometimes just a tap will free them up.
As an aside I replaced my car horn with a set of Hadley five inch electrical horns and they by themselves give a very nice and loud sound, not a lot of money either.
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)