[Tom Ferry]

Definately take the ply flooring up first. If water gets in there via a chip then it rots the wood underneath & eventually your van starts to stink.... Happened to a mate of mine.

Its a time consuming job but its better to do it right first time.

[SherwoodCleaningSe]

The instructions say that you can do either way, I know a guy that has had it on ply for over 5 years and he's happy.  I did it straight on the floor, my reasoning is why stick ply in a van and lessen the pay load.

Simon.