We altered the arrangement under the mattress in the berth with slats made of 1"x3". When only on plywood or the plastic deck mats the mattress really got wet when we were out in cold and freezing conditions.

Even if you make them yourself, slatted wooden bases for beds and settees are much to be preferred over plywood bases. Mine are made from 70mm x 20mm softwood, varnished, spaced 50mm apart, and the mattress and cushions can breathe well. In contrast, plywood bases, even when perforated by large holes, tend to encourage black mildew on the underside of the cushions, when in full-time use.

Ikea sell replacement curved wooden slats for bed bases quite cheaply. They might even be worth considering for normal bunks, as they allow air to circulate below the foam cushions and provide a little springing. They could be easily cut to sit on rails between the bunks.

Outside Europe there must be other companies selling similar slats.

Cheers,  Slieve.

     George , best and simplest is get a sheet of ply thick enough to take your weight cut it in strips about 2 ft wide to bridge the gaps then stoe them under the berths in the daytime , or go to a motorised caravan sales site and have peek at there caravanetes and pick up some tips .  cheers. 

Hi Georg - try this on the KYOA site. It's like the infill board we used.