Upgrading to sprung caravan upholstery

Just be aware that sprung upholstery is heavy and the extra weight will eat into your payload and might even increase the nose weight of your van.

Why not investigate using a better quality/more dense foam? Any good upholsterer should be able to help.

Don't know where you are in North Yorkshire - but I've used Ian Watson in Bridlington in the past. They are an upholstery company who specialises in caravans.  We replaced our useless foam cushions in our then new Barcelona with a high quality high-density foam which he said should last for 10years.  It cost ( about 7 years ago) £250 to have both front seat bases done and they were completed in 48 hours. As TW correctly points out, though - even that upgrade came with a weight penalty.

They did say that flimsy seat cushions was a problem in most modern caravans - regardless of make.