I helped a Westy owner friend "upgrade" to a custom built Sprinter.   Total cost was just under $60k, including a low mileage 06 cargo sprinter (06 was the last year of the very fuel efficient engines).   A lot of $$$, but a nice machine (and cheaper than some GoWesty Westies)

This big rig gets 22-23MPG on Diesel.  TONS of usable room compared to a Westy.  it's a much larger vehicle, but easy to drive.  You can see how we borrowed a lot of ideas from the VW Westy (kitchen on drivers side, rear seat bed etc).

http://californiavan.com/sprintercamper/

Contact me via Pmail if you need more info...