As far as the rail height goes, here is what I have done for my set up. I have a 2014 Ram 3500 dually with the fifth wheel prep package and a Reese R-20 hitch. My 5er is a 41ft Raptor toyhauler. I use the Reese picture frame adaptor in the bed of the Ram and that raises the hitch 2 inches. I also have the head of the hitch itself set on the second to the highest mountung setting. That raises it another two inches. This setup allows me to hook up and tow very level. Once I made the hitch adjustment my mpg while riding went from 8.5 to 11.8 on the same route. When unhitched, I can still close the trifold tonneau cover with no interference. When hitched up, I have 6.7 inches clearance between bed and bottom of trailer.