"The side of the van curves inward, but a one piece shower surround doesn't, so it wouldn't fit."

True, that's why I used the shower pan only and built my own walls.

32 inches would have been too wide for me. My shower / bathroom is directly behind the drivers seat.

Consider a 32x30. Good luck with your build.

 

I have found that a shower curtain is not required. Did not want a door due to space requirements or a curtain due to it blowing about with a van window open. So what I have evolved to is a odd method of showering. First was to eliminate the spray shower head. Now use a radiator fill valve with full flow. A spray head causes water to bounce off the body into the aisle. So first step is to put down a towel in front of the shower. Next I kneel down and stick my head into the shower between the portapotti and the shower wall. Clean my hair. Then step into the shower to get wet without a spray and soap and rinse. Very little water gets out of the shower onto the towel.

Not like a shower at home but you do get clean.

 

https://bit.ly/2GnZcDi
Should be cued up to around the 13 min mark where the shower portion is [folding DIY]. Recommend watching the whole thing for ideas.


You can also open the rear doors to 90 degrees and magnet or tie a tarp around it as seen here.
https://bit.ly/2GlGnR5
Many videos of such DIY solutions (more rudimentary than upscale) in the account above. Geared more towards wilderness vs urban environments, obviously.

 

Like Transit-Paul we also have an actual bathroom in the van, including a Laveo Dry Flush and a shower kit from Lowes. (T-P, I can't figure out how you're using trash bags as a replacement. Please let me know.) Except ours has an east-west orientation. I made the shower pan and it drains through the floor to a gray water tank. Shower is closer to 26 x 29 but still ample size in our 148" MR. And to keep Orton happy it serves multiple purposes - since we have a teak "raised" shower floor that dries quickly, the dog bed goes in there at night. Everybody's clean and everybody's happy!

Regarding the curved van walls vs the straight shower walls: I framed the bathroom to have straight, rectangular walls. One of the voids is used for storage (accessible through the rear doors) and the other has a medicine cabinet built in. May no square inch go empty.

 

"(T-P, I can't figure out how you're using trash bags as a replacement. Please let me know.)"

Not a exactly a replacement. I Keep the cartridge in there ready to use. Lift up that large white trim piece. Put a bag in there put the white thing back, do your business, take the bag out tie a knot in the bag.