I tow with the same motor and I don't worry about using high RPMs to get our 9K# rig through the Mountain West region of the country. I find ultimate power is available in the 4K to 5.5K rpm range. We predominately camp the four corner states and live at nearly 6K' elevation making all of our towing at high elevations.
Although gas engines suffer a 3-percent loss per 1K', I just rev her up and she always pulls us through. If I have towed through the area before I anticipate climbs in elevation utilizing momentum to minimize extreme RPM scenarios, usually have no problem maintaining posted speed limit. I do have a scan-gauge to keep an eye on tranny temps.