(1) No need to go to the roof, only airlines at the horns and since you said they eventually sounded we know the air and horn part works. Mud daubers would only change the volume and/or tune. So, back to the inop part, your correct there should be a relay AND an electric air valve, your wire at the smart wheel most likely completes the circuit to ground and closes the relay sending power to the air valve. your chore is to locate the air valve and start trouble shooting from there.
Rick & Becky SKP129417
2018 Ram 1500 Experimental
2018 Airstream Flying Cloud 27FB