If you have air horns, it is likely that they plug into a air manifold in the front. Ours is mounted on the "firewall" in front of the driver. The problem most people, including me, is recognizing it for what it is. Mine looks like a small sprinkler head with threaded plugs in it. When I started, it didn't have anything plugged into it.
The best setup for filling tires is to add a small nipple to the manifold (after you are sure that you have drained the tanks with the lanyards), and then put a brass roller valve on the nipple. This way, your system can be sealed tight. I then put another small nipple and a brass quick disconnect. It works great. I realize that some higher end coaches my already have a setup like this and I've heard about some of those being around one of the front wheel wheels.
2000 Georgie Boy Landau 36' DP
2005 Saturn Vue toad