The chassis itself of the F-150, or that of the upcoming 2017 Super Duty, is made of high strength steel which saves a little weight. Mostly what Ford accomplished with high strength steel is a much stiffer chassis by being able to change the design. I believe it's fully boxed now which should be a lot stiffer and lead to better ride and handling.
Most large trucks on which motorhomes are based still use chassis rails made of open "C" channels which aren't particularly stiff (especially in torsion). That combined with extreme lengths reduces chassis stiffness. By modern automotive standards it's hard to expect these motorhomes to ride or handle well compared to automobiles. There is definitely a lot of room for improvement, although it would add cost. Maybe Ford will eventually use the upcoming Super Duty chassis for smaller stripped chassis to replace the E-Series.
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