I ordered in a Zinus brand memory foam bunk mattress for under $100. Have same brand in a short (RV) queen on the master bed. We like it a lot. Might be called a narrow twin mattress.
2019 Cherokee Wolf Pup 16BHS flipped axle
2019 Ford F-150 S-Crew 5.5 bed V8 w/tow package, TBC, Tow Mirrors, 3.55, 7000# GVWR, 1990#CC
Husky Centerline TS WDH 400-600# spring bars