Pop up mattress

I would look for a foam topper or thin foam mattress on Amazon. Sizing probably won't be exact but get a king and cut down as necessary.

Thats what we did with our 2008 Starcraft 14 R/T Popup trailer. Got a 3-inch topped from on-line just for the bed we sleep in. Does great except when when the temps inside get below 32 degrees inside the trailer. Then the FOAM gets hard as a rock... Even sleeping on one and not running any heat inside sometime ends up with a silhouette depression in the topper. The foam pillows do the same thing haha...

Of course it is rare to get below freezing inside the POPUP trailer but it has happened on a few occasions...

The 3-inch foam also fits right up flush with the lowered roof when in transport mode. Was never a problem but just wanted to pass that along... You would not want a light to turn on when the roof is down for sure... Especially if your ceiling lights use the very hot incandescent type bulbs...

I also covered the floor of the tent beds ends with those two foot squares stick down carpet pieces... This really helped the tent bed area keeping it warmer. We also use those 2-inch by 30-inch foam strips that come with house window air conditioner kits to go in between the outside flaps and the mattress all around both tent bed ends... This cuts down big time on any cold drafts that might come in under the POPUP canvas ends...

Roy Ken