I wonder why no one has tried to answer you? What is wrong with the floor? If it's become soft due to the "sandwich " separating, there is a product on the market wich can be injected through the plywood.
Depends on how bad the floor is? I know as i have replaced the front end of my caravan.First thing to do is find out where the leak or cause of damp is from? You can buy what you need but the cheapest way is to make it out of wood. I know you wont get the same insulation properties but i dont think that really matters to much for the time you use. I would say to check the wieght before you reuse the caravan though as it will mean the less you can carry on board wilst towing. Mine worked out as 10kg over just ment i put less in the caravan.
Steve