pjay --
I'm assuming that you already have lots of insulation under the finished wood, if not add more that minimizes conductive and radiative heat loss before continuing, then it depends on the type of mattress you have.
I went with a closed cell latex foam mattress, much warmer in winter. No possible vapor migration that causes most of the moisture under the mattress and resultant mold. With a closed cell mattress you can place it on the finished wood and not worry.
If you use your camper in cold weather, where condensation is an issue, and have a conventional mattress or a open cell based foam mattress, use the some what pricey Hypervent matting under the mattress. This is what came under the mattress in my sailboat (available from Defender Marine). It is very effective at dealing with the moisture that is migrating through the mattress from you as you sleep. Mattress stays dry and mold is less likely on the colder camper surface.