I would do the test drive with my RV attached and them in the passenger seat. Not a chance I would let them do it themselves.

 

Have you gone into the truck display feature and ensure that the truck is reading the right type of brakes for your RV, i.e. electric, not hydraulic ?

 

That mesage doesnt necesarly have anything to do with the trailer brake it is most likely the switch on the e brake. try applying the e brake and realeasing it and slip your toe under it and pull it up and if that fixes it have the dealer replace the switch