ABS light

Maybe not related at all, my old '97 F150 would experience that intermittent ABS light, but almost exclusively when it was wet and cold outside. Never any trouble found by various mechanics looking at it.

Maybe you'll get lucky like I did and Yes, pretty hard to fix something when its working, then later when not at a shop it messes up. On mine (that is different make and model) I found everything I could on-line, and also found out there are many reasons why an ABS light goes off. Then I started with the simplest first. The simplest for me was following the wires and at every connection make sure the connection was tight. I found a loose connection right above my diff cover. A loose connection as in when I pressed the connection together real hard, the ABS light went off. Then I put a big hose clamp around it, waiting until I ordered a new connection and had time to put it in. Well that was several years ago, no more problems. Thanks for refreshing my memory, maybe I'll get around to ordering another connection?

Which Chassis do you have? I take it, a Workhorse since you had it looked at by a Chevy dealer... But more important to know which Year/Make/Model of Chassis vs. the Coach to deal with an issue like this. That said, I guessed W22 with 8.1V8. Then tried to at least look up some parts. RockAuto.com shows Workhorse, and their selection for braking was very limited and nothing for electronics. So no joy. A 2002 Ford F53 chassis has a sensor on each front wheel (close to the hub) and one sensor at the top center of the rear axle (above the Ring Gear). Working on our Fords, I've seen dry "dross" clinging to the Front Sensors, and greasy "dross" on the Rear Sensor. This is because the Sensors are mildly magnetic, and the fronts are out in air, while the rear gets axle lube slung onto it.
That "dross" is from wear on the rear and wear along possibly with rust on the front. Normal if it's not excessive, but too much will bother the ABS "brain" and put the Light on. If it was mine, or we were together and I could help you, I'd try to pull the Sensors and be sure they don't have Metallic Stuff clinging to them. Again on Ford, the Sensors are held in place with little 10mm fasteners. The Rear has an O-Ring Seal, and since it's into the Axle Lube, I clean the mounting area with spray brake cleaner before removing it so dirt doesn't fall into the axle.
I don't think the Sensors can be tested with ordinary tools like a Digital Volt Ohm Meter (DVOM). On Ford, I'd risk $10 on the Rear one, but the Fronts are closer to $100 each.
Again on Ford, the ABS does a "self test" every time you start the engine, so I an understand how it might set the light while driving then not light it next time you start up. On one of our Fords, a failed Rear Sensor turned the Light on and off while driving. It also took the Cruise, Speedometer, and Overdrive with it.

I've got the same issue with my Chevy truck. I'm guessing it is a sensor going bad. My understanding is when the light is on, the brakes work, but the a it-lock is not working.

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Intermittent problems are very hard to find. If there are no codes then you just have to go with it until it fails.