Volvoman
My W900 is 285" wheelbase (singled). I set the boxes at 14".(even with the cab step) I measured several motorhomes/toterhomes. Some were a little lower.Mine is a toterhome so I have almost no overhang. My old motorhome had a 10' overhang with 18" of ground clearance and it would drag on almost every driveway you pulled into. I would suggest the boxes behind the rear wheels taper up to 24" or more of clearance. MMM