My educated guess is that the EVC was delivered to Winnebago with the window in the slider already. Otherwise why would Winnebago install a non-opening window there and opening window elsewhere in the EVC. Especially why did they install an opening window above the refrigerator exhaust opening? I would suspect that the difference in the weather stripping is because of the different style of window and not from different window openings. But until I can get a look at the Weekender in person or someone with direct experience can comment, it is just an wildass guess. Thanks for your comments.
The vans were long wheelbase panel vans (they received exclusive rights to these in the US, I understand), and Winnebago cut the door for the window.
The story goes that Winnnebago wanted a platform for the Rialta, and VW wanted a camper for the American market, so a deal was struck. One of these days, I need to research this for accuracy!
As for why Winnebago did this, if you own of of these, you know that Winnebago did some bizarre things when putting these together (can't open the undersink cabinet when the bed is down is just one of many....)