Have you seen this thread?
http://ecomodder.com/forum/showthrea...d-e-18151.html
Optimal Boat tails are more organic (ie. curvature) than a single straight angle.
Starting the tail inboard from the edges a few inches will work, but isn't optimal -- you'll lose some energy to trapped vortices/turbulence in the "gap" between the RV and where flow re-attaches to the tail.