Ikea do slats which would go onto the bed board,though whether you could still fold the beds,I dont know. You can also get slats that you can roll up,i have them on mine.They were expensive,though actually cheaper through camperlands than from the manufacturer. An other option is a froli bed system,again expensive.If you are handy,I am sure you could replace the entire board with slats but it seems like a big job that could go horribly wrong! Some of those cheap bedrolls from Go cut to size and stuck down on the bed board may help.
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We had the same problem in our Countryman. We bought a drymat from force 4 chandelry. It allows the air to circulate under the mattress reducing the condensation. Its light and can remain on the bed board when packing up. Cost around £50, but well worth it IMO. Sorry I don't know how to put in a link.
My stay down,so it's been years. You can roll 'em or fold 'em though.They would probably ok on the bed ,our old conway had an extra topper that was as thick and that was ok. If the beds comfy already then Jinty's idea looks good.
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Duct tape is silver