You are correct Bombsight, the mattress fits very snugly against the walls - as long as the ambient temp is not too cold it probably is not a real problem. But when you get two people sleeping in the closed space with the furnace on to help take off the chill, you will get some condensation on the walls or floor, and it may freeze temporarily. I speak from experience here. Maybe I didn't open the windows enough, but with the temp in the mid teens, opening the windows wide open wasn't that appealing.
I wanted something open underneath the mattress to allow air flow, and I made my platform/ spacer with 1x2 cedar boards on edge - ~44 of them 1/2 inch apart held together with 1/4 in poly rope threaded through them. I can roll the platform up in a cylinder like you roll up a large map. The platform raises the mattress 1.75 inches off the floor. Large enough space one could hide some items there if they needed to. I will see this November how they do in Goblin Valley, which should be chilly again by then.
I did go out and measure the living space as I said I would
The cabin measures 53 inches wide and 79 inches long. The inner height is 45.5 inches at the cabin doors. The height in the back of the trailer, beneath the cabinet and the kitchen area is at a minimum of 20.5 inches tall where a support beam goes under the cabinet. Other than the beam you have two or three more inches. Your mattress will be 6-8 inches thick depending on which version you choose. On mine I will have to add 1.75 inches due to my platform I built to hold the mattress higher. The distance from the floor to the lower edge of the cabin door sill is 4 inches. So the mattress does sit higher than the cabin door margin.
Interestingly very little dust or grit made it into the cabin despite being in a convoy of 6 or 8 vehicles on the road into Cathedral Valley in Capital Reef. The rubber gasketing seems to work pretty well as we were in a veritable sandstorm cloud from all the vehicles in front of us. And the mattress sits tight enough against the door sill that ingress of dust and sand is minimal.
On my trailer the distance from the ground to the lower sill of the cabin door is 31 inches, and my trailer sits on BFG ATs that match my FJC. Hence the need for a step for my spouse - or at least she prefers a step be present.
