If the brakes were operating correctly before and this problem just occurred without anyone messing with the front brakes, it's possible (as Gerald pointed out), that the calipers are not retracting due to mechanical binding. There is a rubber "O" ring around the caliper piston that if destroyed will add friction to the caliper piston. This occurs quite often if you had previously gotten the front brakes very hot during a long highway descent.
The amount of fluid would not be the problem unless someone added some recently. You could bleed the front brakes and see if the calipers return to the retracted position.
If the coach has been driven through water, it's possible that all the lubrication on the pins that the caliper slides on may have rusted and there is too much friction to return.
Unfortunately, taking off the wheels and inspecting the brake system is the only true way to diagnose this kind of problem.
Good Luck
