I assume you know this, but Styrofoam (the white stuff) also absorbs water.....and it takes a very long time to dry out (much longer than spun glass insulation). Of course, if it doesn't get (and stay) wet, you'll probably be fine.
Why not use thicker foam (or multiple pieces) where you have the room and thinner foam under your tanks?
Under my tanks, I used a piece of 1/2" camping mat with a piece of 1/2" plywood underneath to help support the tanks (the plywood is supported with tubular steel that bolted up to the original tank supports).
I also cut lots of small foam pieces with my bandsaw to fit as needed. Glued them in with spray adhesive. Note that most spray adhesives will eat Stroyfoam, but are fine for Blue Board (or similar products designed for below grade use).
Don't forget low point drains to get any water out that will eventually find it's way in.....esp. if you tow when it's raining.
Great project and I suspect you'll find the floors warmer on your toes when done.
