On our last Fleetwood Discovery, the air horn trumpets were mounted under the coach near the electric steps. Worked well there, and didn't have to worry about roof leaks. As a side benefit, if my co-pilot started to fall asleep and my drink was in need of replacement, a quick toot of the air horns directly under her would wake her up in a hurry!
Buying a kit should give you everything you need. Just wire the air solenoid to the existing horns and tie the air line into the air service port you should have up front.
Good luck and post pictures of your new project