So, my shower floor went through its third iteration last night. It cost me about $50 in materials from Lowe's (not including the majority of a 6 pack from across the street) but that price came with explicit presumed free use of their parking lot for the project, so I considered it a fair trade.
1) When I bought the van, the bit of floor covering the shower basin was this foolish folding thing made of low quality plywood, vinyl, and metal trim. It was quite good at soaking up residual moisture from the shower and cultivating growths. Also, because it rested on top of the tub, it made this piece almost 1" higher than the rest of the floor, creating the least headroom anywhere in the van. Not acceptable.
2) For a temporary solution, I found these corrugated plastic bits and used them to fill in the space along with some rubber matting underneath that would allow drainage. It was light, not mildew-incuding, and level with the rest of the floor, but never intended to be a permanent solution: The corrugated bits let dirt fall below, the rubber mats made it difficult to clean, and it just wasn't a tight setup in general.
3) I spent a while thinking about what would make a sturdy platform, but not allow moisture wicking materials to contact the shower floor, and also prevent material from falling into the shower basin when in place, yet still be flush with the rest of the floor so as not to reduce head room... I settled on a design using plywood and PVC. The diameter PVC pipe I chose created exactly the right height to make the top of the plywood flush with the floor. And of course, PVC doesn't rot or wick moisture.
Then, I used some rubber flooring material that hooks together to create the right dimensions... it's only 1/8" thick so it didn't cost me head room... and the snap-on edges cover the gap between the floor and the shower preventing crud from falling in there when it's not being used as a shower.
...and after trimming everything in place I have a new shower floor! Much nicer than before. Start to finish ~2 hours.