What age is your Dandy Brian.
A few years ago we were at a Dandy meet and an elderly couple had revamped their old van and it was lovely. The had a brand new mattress made for it and gave the supplier the exact measurements. He thought he was doing them a huge favour and treated them to a deeper mattress. This was just before the meet and they had disposed of the old one. Needless to say the deeper mattress stopped the Dandy from folding so the mattress travelled all the way to the meet on TOP of the trailer.
Are you talking the flat bed mattress (not the divided settee one)
I should think comfort takes priority so a foam one that suits you density wise. Don't forget that the mattress usually gathers some condensation underneath over night so the mattress should be aired (turned) frequently whilst camping. We find though that hardstanding is better than grass and doesn't produce so much moisture.
We use two Thermorest Ridgerest So-lite camping mattresses under the main mattress and it keeps a lot of moisture off. Not the cheapest thing on the market (Amazon have them for £22.47 each) but we have been very pleased with them. We know they work because they aren't an exact fit and we get moisture on the base board in the 2" gap in between the two Ridge rests.
