Have you checked ALL of the ABS wheel sensors? One can go bad and it will keep the ABS light on at all times. Most have a pigtail connected to them which can be disconnected for a resistance check. On my coach, the normal reading is 1200 ohms. Anything other than that is a bad sensor. On your coach just look for the one sensor that reads differently than the others.
other than that, there are several RV salvage yards around the country that may be able to supply you with a working ABS module. I would also guess that many truck salvage yards may have parts for an F53.