Hi Chris and welcome to the forum. The extent of the job required could vary considderably and you will probably only find out how bad things are once you start to remove the affected rot to get back to sound wood.
In the best instance, it's only affected area is the bottom ply, in this case, after making modifications to stop it reoccurring, cut out the rotten stuff back to sound ply and patch with new ply. You may need to add a batten behind to fasten the new onto the old.
Next worst is that it has rotted the wooden battens that make up the side wall and floor construction. Or worst still it is rotten right up tp and including the floor ply. The meathod is the same in each event, cut out, to sound wood, dry, fill in the odd missing area, treat with a breathable wood preserver like Cuprinol isn't a bad idea but let the wood breath so no underseal or gloss paint only emulsion, like original.
As said, no point starting it if you can't stop it happening again so find and rectify the cause. It's fairly basic woodwork, you won't find any dovetail joints in a caravan's bodywork so hopefully it won't be too bad a job, but let us know how you go on. Good luck.
Dave.
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