First trip away this year was a gentle reminder that I still needed to install the planned insulation below the sprung mattress in our 542. Being a fixed bed we suffer a large volume of unheated cold air below the mattress. Normally not a problem but on the coldest nights its clear that it has a big cooling effect on the van.
Solution was a trip to a builders yard for 2 thermo insulation boards 1.2m x 2.4m x 25mm (£40 delivered). These are polyurethane (and not polystyrene) and can be easily cut with a kitchen knife. Offset the join away from the middle as this is where the mattress join is. We never access the storage area from inside the cabin so no loss of utility. The acid test was rolling on the reinstalled mattress to see if it creaked and flexed - silence, that's a good start.
It may not be the total solution but it's worth a go. If this doesn't have the desired impact I will splice in a hose to the blown air to push some heat into the storage area.
Pity I didn't do this before the winter.

Thermo Board.jpg - Bild entfernt (keine Rechte)

Be careful, doing this can cause damp and mould on the underside of the mattress if going away long term, this stuff helps stop it as it allows the underside of the mattress to still breath.
https://shipshapebedding.co.uk/shop/dry-mat
Dave

I'd agree with the 'naut, condensation onto solid surfaces under and around mattresses can be a big problem, that's why they're set on slats to ensure there's plenty of ventilation.

Aren't the boiler/heater in the space under the bed? The residual heat from them should keep the space relatively warm.

We had an Adria motorhome where the boiler/heater was in an open shelf off the 'rear' garage under the fixed bed. It kept the whole garage area warm, indeed the frost stat switch that vented the water if near freezing was positioned close by so water wasn't ejected too early. Mind, it also had narrow ducting tubing running right the way around the edge of the mattress space too.

Thanks, yes all valid points that you raise regarding the barrier causing a moisture trap. I did consider this in my planning and satisfied myself that for the number of days in the van, combined with the fact we remove the mattress between trips etc it was a worthwhile risk. I think we will be happy with the warmer nights. I will let you know if we hit any problems. Thanks and happy vanning.

Currently on our NE Scotland campaign and a good time to update on the insulation. As you may know the underbed storage area under 542 fixed beds is huge and completely unheated. The insulation board has been a revelation in that now the sprung mattress is at the same temperature as the cabin, not the storage space. The mattress is no longer cold to the touch during cool evenings and mornings. So, the conclusion is that this is a big success and made for a more comfortable sleeping setup. Recommended.