I have to note, that I have been doing it for 3 seasons, to every coast (~12,000 mi), over many major mountain ranges, and have yet to be disappointed.
Yes, you do have to correctly size the fifth to the vehicle. And the OP's seems to be oversized. But to categorically reject the combination is a little too broad of a conclusion. It can work, and work well.
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder