Does the rubber washer on an outdoor conduit whip go inside the box or outside?
The first:  

Whip -> Washer -> Hole in Box -> Nut (rubber washer outside box)

not 

Whip -> Hole in Box -> Washer -> Nut (rubber washer inside box) 

You want the plastic washer on the outside for a watertight seal, and the nut on the box for mechanical strength and with a metal box, to bond the fitting to the box electrically.