Hi Jupiter 53, sorry to hear about the spongy floor of your caravan.
This happens to many vans and whilst unwelcome, can be fixed by a handy person fairly easily.
There are generally two methods of approaching the repair, one is to lift the floor coverings and repair from the top using gravity to assist the bonding adhesive to penetrate and rebond the upper plywood to the core material (often a foam) and thence to the lower plywood and so make all 3 component parts, a fully integrated floor of considerable stiffness.
The second method is to approach the job from underneath the caravan ( you may need to raise it using car ramps if ground clearance is tight or if your girth demands it!)
As using this method means that gravity is working against you so to speak the adhesive needs to be introduced to the holes you drill under a modicum of pressure for which a special "gun" is needed,,,( fortunately they are quite inexpensive)
Our own Sir Sprocket has photographed his own splendid repairs using the underneath approach and also provides a detailed description.
Have a look here for more details.
http://www.practicalcaravan.com/forum/technical/49042-floor-delamination#382274
Good luck and I hope the repair goes well for you.
Regards
paws