Wow, you're boat looks a lot different than mine!! I have longitudinal stringers and then there is like a platform on each side that the tanks sit on. No foam. There is a rubber mesh that the tank sits on and they are just strapped down. I guess the rubber mesh is suppose to allow any water to drain if any got in the tank area.
Regarding MDO, I have used it before, but I don't think it's a good choice for the gunnel. Its basically just plywood with an overlay of a paperlike material so that it paints well. I would use either a marine plywood, or go composite with coosa or penske board.
My first choice was coosa or penske board. what size tanks do you have and how high are the stringers ? I was planing to coat the tanks with cold tar epoxy than foam in tanks at the ends and the middle .