I'm about to take a travelnurse assignment over the winter in WA state and have come up with something similar for my Astro 4x4 which isn't big enough to fit a shower and also as a permanentish shower shed for my off-grid property 150 miles away. Note, this isn't "needed for camping" although it may be useful there as well, it is "needed for maintaining daily hygiene while living in your van and working in a healthcare facility while it is freaking cold and wet outside all the time".
I'm just doing a 4x8 x 6 box out of wood or metal framing, 2" foam, and glued and heavily-painted canvas cloth or fiberglass window screen. This can be towed on a trailer or mounted on my hitch rack and the size can be adjusted down as needed. Just a very simple setup with a bucket toilet (composting to come) and a shower with a propane on-demand heater and perhaps a simple sink for face-washing etc using the shower hose, or maybe like in my TT, a selectable faucet. Lexan roof to provide natural light, and a safe place to hang my Buddy heater. floor would be perforated rubber mats over steel mesh or wood with gaps. Can mount the license plate and some cheap trailer taillights to the back along with a high-mount stoplight and plug into the trailer light receptacle.
This was my original concept although I may scale it down some...or maybe not, might build a mockup first or I might not since I'm only going to have a few days between the time I fly in and start work and will be showering at a gym until I get this working. I also don't think I'm going to worry about fairing it in, just make sure it's far enough from the fan rear door to open all doors which will give me some privacy while I step between them.