A few thoughts in no particular order...
1) Not sure why the plywood goes under the polyIso...I'm guessing you're looking at all the sheetmetal indentations on the floor and wanting to even them out? I'd consider gluing the polyIso directly to the metal floor. If you make liberal use of a product such as
Sikaflex 221, it will actually dry in low spots to provide support to foam on top. If use adhesive sparingly, I believe the polyIso will compact into low spots and provide even support. Both scenarios will have best chance if the low areas aren't more than 1/4" to 3/8" deep.
2) polyIso will lay nice and flat for you, allowing easy use of boards/weights to hold it flat while drying. 1/2" plywood will have a mind of its own to varying degrees...will need a plan for screws and/or weights to hold flat when adhesive cures.
3) most people don't insulate van floors and those that do rarely use 1" plus all the other stuff you plan. I'm not sure I understand your comment "I do intend to have air circulation, so I'm guessing floor insulation is needed". If you plan to run AC in hot climates, floor insulation makes sense to a degree, but "air circulation" doesn't quite compute as reasoning...
4) resilient flooring like vinyl or wood makes for a much easier to clean area...can always use throw rugs.
5) Here's my floor buildup: It's 1.5" polyIso covered with 1/4" luan, which will then be covered with resilient flooring. All glued together with Sikaflex...
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