I have a Chevy 3500HD diesel. I am in Colorado with a lot of steep downgrades. I pull a 17,000lb fifth wheel. I am new to all of this. I had been using the manual selection on steep downgrades......running usually in second gear. Now I just discovered I have an exhaust brake button I didn't know about. When this is engaged, and tow haul is on, do I leave the transmission in drive or still use manual to keep the truck from gaining too much speed?