I want a Roadtrek 190 Popular van already fixed up. Have been looking at these things for five years or so. There's a handheld shower above the toilet and a drain in the aisle floor. You pull the accordian doors on either side of the toilet out into the aisle, pull a shower curtain around and shower in the aisle. They're 20'5" so can fit in a lot of parking spaces.
For DIYers, this shower gizmo is worth its weight in pure gold. It is well made, the pressure is awesome, and takes 4 "D" batteries that lasts a long time (can use rechargeable via solar batteries). You only need a bucket of water to submerge the inlet part... I have gotten much use out of mine, and it is vastly better than some of the others sold on Amazon.
The battery pack is water resistant, but I enclosed that part in a plastic bag to make sure it stays 100% dry. Although the picture doesn't show it, the cord connecting the battery pack is fairly long enough to keep it mostly out of harm's way as you shower.