I recently traded a motorhome that I felt had too short a wheelbase with a too long rear overhang. I think it's a case of the interior designers driving the design process. In order to fit the bed and bathrooms in the rear wheel location becomes dictated by the location of bath fixtures and slide locations. I know in the Jayco Melbourne I used to own the rear design was almost identical in three different models which had wheelbases from 181" to 220". The overhang behind the rear wheels was almost the same on the three models.
Mike