The included mattress is about 4". From the mattress surface up to the bottom of the fan is 7" of clearance. Mattress surface up to the flat portion of the cabover ceiling is 9" of clearance and less where the ceiling edges slope down. When the pop-top is closed and fabric folded in, there is slightly less than 2.5" of space remaining above the factory mattress should one want to add a topper or Froli system under the mattress. Froli systems add 1.3" to 2.36" of height depending on which of the three models are chosen. The most common for boats and campers is the Traveler which is 1.3" tall. And, if a person was replacing the entire mattress, one should target less than 6.5", so a 6" replacement mattress would be a good fit.
I went ahead and edited one of my previous posts and added this info.