Mdf is cheaper to buy and easier to work with, its also easier to get with a wide variety of different finishes.
If your tooled up to work with mdf then making something with ply is a pain because its so different to work with.
Mdf not as resistant to moisture as marine ply but as good as ordinary ply which tends to de-laminate when it gets moist. Such high moisture levels should not normally be an issue in a camper, since levels high enough to damage mdf would cause severe mould and rot damage to the soft furnishings and render the whole interior an unpleasant place to be IMO.
The main advantage of ply is that its stronger, more resilient and considerably lighter.