Generally the underside of the floor panels will already have some from of protective coating on them, I'm not sure if adding an even stronger moisture resistant surface might impair the airing of the surface and might encourage rot setting in.
and any wooded beams should already be treated timber to resist rot.
In general floor rot is caused more by water leaking in through the roof and walls rather than from directly underneath.
In storage you shouldn't get "rising damp" as there should be no porous material connecting the caravans floor with the ground or vegetation.
On balance I do not think there is any need to add more protection, unless something has already been replaced.