I read a few posts back that you decided on a particular mattress and i hope that brings many comfy night's.
Just as an FYI for those who may come this way...And i have shared this elsewhere but in context it's good food for thought for those still pondering...
In our Van conversion and our full time rig we utilize the Froli Sleep System
with a memory foam mattress topper from a big box store. The sleep is amazing and rivals ANYTHING on the market for home, RV, or Yacht use. We have been comfortable in the Van in situations ranging from snow on the roof (with no heat overnight, our preference) to 80+ degree evenings with simple fans going and never had issues with hot/cold feeling with the memory foam topper.
This is a picture of the Froli before we put it into our Van:
you just place the system on top of your bed platform and screw down in a couple places so it doesn't shift off platform. On our queen in the fiver i have it screwed down so we can still lift up the bed to get into the storage. It is flexible enough to be utilized above berths with under-seat storage access hatches.
Then you place your thin (~4" to 8") mattress topper(s) on this and you are good to go. The system allows airflow under the mattress so there is no chance of mildew, etc.