I finished my 51 6400 a couple of months ago and took her for a 1800 mile test drive. The family and I went to the California central coast from arizona by way of New mexico, Utah and Nevada. Mind you that this was all with a 28ft 1967 silver streak travel trailor in tow. Below sea leavle to over 9000ft in elivation with no major problems. I have some pics, and as soon as I figure out how to get the into the computer I will post them.