The water tank is the raised area. When the doors are shut on the underbed storage there is a 5" wide area, then the raised area. We store 4 plastic storage boxes, 11" deep and 17" wide, and around 24" long. Made me go to the trailer if you want exact measurements. The boxes fit side by side, with another 2 side by side behind them. We attached the rear boxes with a rope and pull. Use nothing on the floor to guide nor protect the vinyl flooring. The doors are held shut with a shackle, so far the shackle has kept everything in place.
On the 21 this same raised area is a tripping hazard in front of the dinette. Or at least we both tripped on the sharp edge.
Myrl and Gary