Maybe I misunderstand, but did you add rooms with the room tool (Plan-Create rooms / Ctrl+Shift+R)? If not, draw a room. Or if walls are connected/overlapping around a room, just double-click inside the walls. Then you will have (opaque) floors/ceilings.

ok

PS Hiding the lower levels from the 3d view is not possible, it is either all levels or current level + all below.