Your RV dealer is correct: you definitely need a brake controller even though you also have an exhaust brake!
We, too, have the Mor-Ryde suspension with the disk brake and we're using the Prodigy braker controller from Tekonsha. I've read where some people say the Prodigy won't work with disk brakes, but it's working GREAT for us!
I looked back in our records to see what we were charged for labor when we had the Prodigy installed. At the same time we had the Prodigy installed, they also installed the B&W Turnover Ball Hitch and the Companion Fifth Wheel hitch. The only labor charges on the bill are listed only for the hitch install, so it appears the brake controller was installed for "free" (other than paying for the controller itself and the controller wires, of course).
$198 in labor charges to install a brake controller seems a bit high, particularly considering we were only charged $230 in labor for the installation of the hitch AND the controller! I also can't see it taking 3 hours to install...unless it's the first brake controller the dealer has ever installed, but in that case, you shouldn't have to pay for their learning curve!