There is no magic potion to a soap and water spray solution for checking air leaks. It's not rocket science. Just put any dish soap you have, about a cap full into some form of a spray bottle, fill the rest with water and carefully move it around to blend it all up. if you shake it, you'll soon see the results of that, inside the spray bottle, not good. Kind-a like shaking a full Coke can!
Then, since you mentioned your coach drifts down fairly quickly on any particular corner, I might start by spraying down your air bags, all the way around, top to bottom. In most diesel coaches with air brakes and air suspension, there's a few miles of air lines all over the place. And then there's air tanks, valves, intersections and more.
I've had air leaks for years and just couldn't pin point them. Well, finally I got lucky and actually heard one, right after I'd parked the coach in its cave at night, when the surroundings were really quiet. I quickly got out my spray bottle which, is always ready with a soap and water solution and, cruised under the coach and started spraying in the area I might have heard something prior.
All the air joints, valves and more I sprayed, revealed nothing. I then thought, "hmmm, I wonder about the air bags"??? I sprayed my left front one and, as you can see in the video below, WOW, HOUSTON, WE HAVE A PROBLEM!!!!!!!
It ended up that, all of them were leaking to some degree. But, that left front was SERIOUS. I ended up replacing all four air bags myself. Not a job for the old guys like me for sure. But, I managed to get through it. I also found out I had a leaking *Pressure Protection Valve* that's on one of my air tanks. That's easy to replace and cheap too. All the air bags cost me around $880.