This weekend I towed my 2010 25FB Flying Cloud up to a mountain property at 7200ft with a V6 4.0L 2011 Toyota Tacoma 4x4 TRD Offroad w/ Tow Package. We have a weight distribution hitch (spring bars).
The grade is steep and steady, and the Tacoma handled it without trouble. Most of the time the transmission sat in 2nd gear at about 1800-2000RPM which meant we were pulling at about 17-19MPH. The engine temperature gauge never moved, though I do not have a transmission gauge so I don't know how it was doing. With the tow package the Tacoma has a separate transmission radiator, which I imagine was helping.
While the truck handled this successfully, I felt that the full size vehicles are a bit better equipped. I've pulled it before with an F150 and felt I had better control.