Used 2011 F350 6.7. Do I need to purchase warranty?

I bought what I could afford. Any kind of exhaust brake is preferable to creeping down a mountain with a gas truck. I get antsy going over about 45 with my 7000lb trailer pushing me, so its downshift as necessary, run the engine up to about 5k rpm then use the service brakes to slow back down to 20 or so and repeat the process as necessary. Have I been misinformed by Ford and their towing guide about having an exhaust brake? Is it so crummy as to be worthless? Thanks