>I just bought a set of reinforced Michelin X tires for my 86 Westfalia. The
>max tire pressure is 40 psi, yet the sticker on the door calls for 48 psi in
>the rear tires. The dealer (Wheelworks, a large chain in the SF Bay Area)
>insists that they are the correct tires. What's going on here?

The tire recommendation is for a wide range of vehicles, including cars. 
The sticker is for that tire on your individual vehicle 

You have a much heavier van.

I inflated to the door sticker pressure, despite the 40psi on my tires.

Just my opinion.
--Dave