Bruce,
My ABS dash light was on all the time and never went out. I had it checked out and it was a driver's side rear sensor. As Kevin mentioned, the ring and pinion had to be pulled to access the sensor. Had it replaced at Prevost in New Jersey. There are front sensors also, and as I understand it, are not as complicated to replace as the rears.
When replacing one rear, have both replaced. The labor is just a little more to do two as to do one. As I recall, it was around $1500.
The newer coaches have a different system and is not as difficult to change but our vintage coach, 1998 chassis on mine, the rears are a PITA to work with and costly.
Denny