I don't know your coach, but all air-brake systems have either manual valve(s) or automatic valve(s) that are connected to the bottom of the air tanks. I'd look first for the airtank - if you find it, look for a valve or pipe coming off the bottom of it and follow it til you find the dump valve. Sometimes there is a cord/cable that is attached along the bottom edge of the coach that you can pull.
Automatic systems periodically dump automatically. If you've ever had a big truck sitting next to you and all of a sudden you hear that loud "pffffffffffffff" that's an automatic dump.
Most systems have multiple tanks and usually there is one small one that is isolated from the others by a checkvalve (for emergency braking if the main system fails) so look for that one too. It will also have a valve on the bottom.
2005 34' Allegro Bay W22/8.1L
Tiffin Coach Owner's Forum Member