I have a switch on the dash that will allow the horn button to operate the air horns when on and only the electric horns when off. You can follow an air line to the valve and then with someone's help check for 12 volts to valve when horn button is depressed. If you do have voltage but no air horns you may have a bad valve.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7