[Fulltiming]

Tony, inspect your brake tanks for any pinhole sized leaks in the tanks themselves. I had a couple of those that were draining my air tanks and thus the wet tank was following along as it tried to refill the brake tanks. There are check valves at each of the two brake tanks (the ones in the front) that are designed to keep air pressure up in the brake tanks even if the wet tank is empty. Unfortunately, those check valves can stick and then the air will bleed out of the brake tanks into the wet tank which feeds all the air lines in the coach. Sounds like you have checked the front and rear six packs for leaks. Also check the low pressure valve near the transmission and the pressure sensor for the dash nearby. Since your air horns are under the coach, the solenoid is likely close to the air horns themselves. You mentioned you don't have an air ride seat. I didn't know I did until I found the lever under the front of the seat that locks it down. It had developed a leak and rather than fix it, someone has used a pair of needle nosed locking pliers to crimp the air line to the seat. A big thanks to Richard for locating that mess and fixing it back in 2009. The hoses and valves for the sliding stairwell cover are located in the pass through bay in my coach. That can also be a leak source. Don't forget the air brakes valve near the brake pedal and the parking brake air release under the dash. All of those can leak also. I find that if the air pressure in the coach isn't up to about 100 psi, you can't hear leaks well even with the detector (of course my hearing isn't super). I had a split in the plastic case of one of my rear ride height valves which I couldn't find with bubbles because it was so large that it was blowing the liquid away from the valve. The 120 volt pump has several fittings and of course the worst of all is if you have a leak in the engine driven air compressor hose so that much of the air never gets to the tanks.

Like you, my coach doesn't have many of the potential leak sites such as air operated interior doors but it still has MANY fittings and pipes and dryers and valves and likely many push on connectors.

Good luck in your search.