Usually best to remove entire damaged floor and replace with marine ply and apply vinyl tiles/simulated floorboards - not a difficult job unless you remove floor under cupboards.

Possum3, I was leaning towards removing the bed frame as its spongy both sides and at the foot of the bed and also removing the seats as it is spongy under the seat near the table. As its an offroad van it has galvanized metal liner under the floor which makes it a bit hard to evaluate exactly what the damage is or how far it has spread without actually pulling the lino up. I hadn't thought of using lino tiles but most tiles I've seen are usually quite thick and heavy. I will look into what's available in them and go from there.
BP

Moisture/water will be trapped in the metal liner between ply and metal (sub-floor), exasperating the problem. Thin metal under composite timber is usually considered poor engineering, correct method is utilise marine ply and coat underside with waterproof sealant.

Note vinyl is available in self-adhesive sheets, tiles and artificial plank.

Ensure you have waterproofed all leaks - use butyl tape over all joins, apply sealant to all areas under windows and where electrical wires penetrate walls (under light fittings etc.)

If it were my van - I would remove all cupboards, replace flooring and rebuild interior to suit.