First, stop worrying. Loss of ABS function is a concern only if you skid (lose rear wheel traction). Everything else in the brake system is unaffected.
Second, you need a more competent brake shop. The light isn't just on - the ABS computer has detected a fault. It can display a code to tell what it is, but a knowledgeable ABS tech ought to be able to scope it out anyway. Wheel speed sensors and wiring is the #1 suspect. And the anti-skid sensor.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC