I'm assuming (since you mentioned getting the floor loose) that you have one of the newer models, not the originals that were assembled (glued-together with lots of glue) Des Plaines Models kits. The more current are more like early Kato (when the roof was a separate part) or Walther/Lowell Smith cars in terms of assembly.
At any rate, I have a sleeper that I opened up to get the batteries out (the lights weren't working in any case, and with the intention of painting the interior. As to getting the underframe off...
I don't know if you MUST take the trucks off, but I did, after I popped one of the couplers apart while taking off the roof to get at the batteries.
The floor is held by 3 clips on each side -one center of the car side, and one over each wheelwell- (probably not correct railroad term, but the cutout for the truck). I managed by carefully prying and wiggling, but... maybe a toothpick near each wheelwell ala a Kato F unit. There are also clips at both ends that hold the underframe to the interior (and clamp the weight between them). The ones at the battery end of the car are accessible underneath the little circuit board that holds the batteries. There is a tiny screw on one side of the board to hold it in place. Beware of delicate wiring. At the other end of the car, the board is held in place by a screw at the end of the board. No need to remove this board- the car work prevents access to the clips at that end - some gentle wiggling and they came loose. I did not try to remove the lighting units.
It may be possible to get the underframe, weight and interior to come out as a unit... but I didn't want to press my luck.
I hope that was clear, and of some help.
