Brake light comes on on tow vehicle towing camper ???

Should have gotten back sooner.

On-line mechanic suggested error code should be readable while keeping the vehicle running, sure enuf, was able to get to the dealer with the light on, left it running outside service door, mechanic read out the error code Wifi on his computer, turned out to be code turned on by by a brake system part malfunctioning, the fix was covered by a tech bulletin, end of problem!!