Right to cut a long story short, I thought I had a leaky door, but turned out to be lower awning rail thought I had fixed it last year, but I must not of sealed it properly so its getting done again.
This is where I need advise. The wood got wet behind the door and flaked up when drying out so have de-flaked most of it but is down to polystyrene middle in places. How can I fill in the damage to make it as level as possible, the polystyrene is still rock hard so has been unaffected by the moisture
I suspect it was not the bottom awning rail but in fact that previous owners had left the door open when it was raining and the carpet had got wet and the floor had rotted from there.
So I used a workmate power scraper and lifted 2 of the 3 veneers of the top ply, leaving 1 veneer.
I then cut a cardboard template of the part that needed replaced and from that cut a sheet of aluminium that I had knocking about.
I glued this to the floor using Gorilla Glue.
I placed heavy weights on the aluminium until the glue set.
Replacing the wood with the aluminium spreads the load better than wood and it also means that should the carpet get wet again, it will do no damage.
I had the same problem and did as Grampian suggests. I scored through the top layer of ply (in my case level with the fridge cabinet) and carefully removed the top layer with a scraper. While it was exposed I took the time to put in some epoxy to make sure the foam was securely attached to the bottom layer of ply and then cut and epoxied in a new top piece. Supported underneath as in a delamination repair, put weights on top and left it for 24 hours. It was solid as a rock afterwards. One thing I will suggest is that when you have the top ply off, check the frame timber at the edge of the floor in case it has started to rot. I was lucky mine was just wet and dried out well.
The quick fix is IsoponP40 which is glass reinforced car body filler. Clean up the area & ensure it is dry with no loose material, then knock up a bit of P40 & spread using flat scraper. Repair will probably be stronger than using replacement wood & certainly quicker. It starts to go off in about 15mins & is hard in an hour. Sand it smooth once hard if you like.
I have used P40 to repair floor on mine including corner that was rotted. Build up any deep holes in layers.
In my opinion, because the top surface of the floor is only 3 ply thick, it was never meant to be "solid" surface, but to give a little under the weight of feet.
Hence my preference to use materials that would also "give" a little, but still spread the load.
Gorilla glue is put on with one surface wet and the glue reacts to this giving an expanding foam bond.
If you use a bond that sets hard, it will lead to cracking between the old and new surfaces.
Its worked on my caravan ok, I repaired it about 3yrs ago. The stuff is at least as flexible as the plywood, its so easy, takes 20mins. Why not do it the easy way? the object of the exercise is a working caravan, not craftsmanship.
if i did it i would put some wooded bearers in.. the idea being to to take the load off the foam and transfer it down to the bottom layer of ply.. wood or filter just sat on foam will never be very strong..
you could just cut off some lengths of broom handle to the right length and insert those into the foam..