Are the floorboards delaminating though? Damp got into them?
If they are you need to drill holes to get the glue into them and then squeeze the panels back together.
Adding 25mm or 50mm foam sheets to the underside and then box in with sheets of ply will add insulation.
Just make sure that you dont block heater vents and gas dropouts.
The static van that we had was planked. IE, not a laminated one.
When 2 of the planks got broken during transit. I put blocks of wood under the broken bit and then jammed a bit of stout wood between the chassis legs. This did the trick.
If your floor is planked, then extra support using this method is the only way I can see of fixing your problems.
If you floor is laminated, then Grampians method is the route to go.
We've just bought a block of insulation from Wickes for about £6 and it's about 48"x 18". this should cut into about 4 or 5 blocks which are then knocked into place. We're then going to use old curtain rail to hold in place and it SHOULD work. Only bought today, so fingers crossed.
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