Every MH can be different. On my 2005 Fleetwood Excursion 39S, the (brake) air tanks do not drain when I release the suspension air bags. The brake air tanks will hold pressure for weeks. The air bags and brakes have to be different systems. If taking all of the air out of the suspension also drained all of the braking air, you'd have no air left to operate the parking brakes.
As Racerguy noted, the air bag dump is rather slow. Sometimes when I sit in the driver's seat I'll line up the dash with something on the ground in front of the MH to see if it is actually lowering. It does, but it's slow. If you want to be sure, use a tape measure before and after.
Bill