Mine slows a bit going uphill also but only about 4-5mph then it downshifts. Either way your not going to hurt it by downshifting, if you feel its starting to pull hard or lug a bit then its time to downshift. If I'm in a hilly area sometimes I'll drop it into 5th and leave it there until things level out. I also tow in TH mode, full engine brake and with cruise set. My tow RPM at 65mph is around 1800-1900, 5th gear doesn't increase it much.