I laid 1/4 in wood lath to fill in all the low spots, then put a 1/4 interlocking sealed foam, 1/4 IronPly and topped it with the new 1/4 LifeProof click lock vinyl flooring (sold through Home Depot). That flooring looks great, will never come apart, and seems virtually indestructible. Has a lifetime residential and 5 year commercial warranty so it's pretty tough. But.... while it's not slippery to walk on, stuff slides around if not tied down. So, I ended up putting down a rubber mat for my dog and tools, etc.
Honestly, I don't think securing that floor would be necessary at all since it is basically 4 layers housed around the wheel wells and follows the van wall contours. It's not going anywhere. Perhaps an issue if I rolled the van, but then I'd have bigger issues to worry about

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However, I did re-install the floor tie downs so I could use them for whatever.
I'll post a picture later.