I have just read the first post properly.
Plyling on the side is very thin, so I very much doubt that you will be able to screw it to that. The only thing you may be able to do is secure it to one of the steel vertical or horizontal cross members behind the ply that would hold the weight.
Better place would be on a bulkhead, but it would need to be fixed were you could get to it easily for resin changes.
We process our water via r/o, through a di vessel and then into an IBC holding tank. This way we have pure for each of our vans without having to carry a di on each of them.