I would dump air and support chassis with wood blocks or jack stands so it is safe to go under. Then remove one of the lines coming out of height control valve (HCV). Start engine and build some pressure. Is air coming "out" of HCV going to air bag? If not, see if there air going "into" the valve. Then loosen the adjuster bolt and move the valve by hand, does air come out?
If both sides of coach won't raise, it is probably the air supply (on my coach a single air supply line tees off and feeds both sides (both HCVs). But still could be other things.
The above is assuming you do not have an HWH air leveling system. If you do, then the air coming out of HVC goes into the HWH manifold before going to the air bags. In this case it could be a faulty solenoid (bad coil) on the HWH manifold (not opening to let air through to the air bag). I have this exact problem on my coach now.