According to a statement a Ford executive made to RV industry recently, the Econoline is the highest volume RV chassis. Most are Cutaway chassis, for Class Cs, with a few being the stripped chassis (as far as I know presently used only for Axis and Vegas).
If Ford planned to eliminate the E-Series any day, I doubt they would have gone to expense of upgrading to 6.2L V8.
Another fact to consider is that while the E-Series is indeed very old, motorhome demand has shifted significantly from Class As to Class Cs, which are enjoying a higher growth rate.
Ford is no doubt pushing the lighter-duty Transit as a replacement, but in my opinion the much lower weight carrying capacity will limit demand. The lower GCWR doesn't help the Transit either.