I've had challenges with some of this type of car over the years as well (haven't by any means solved them all).
It's possible that you have to remove the detailing from the bottom (often attached via screws to one large plate with all the details) and then after that there are the screws that hold the frame to the body as Nicole suggests (some may be under the detail plate so you can't see them until you remove the plate. As Nicole said, be careful with the wires. Unfortunately, there is not a connector between the rollers and where the wire snakes up into the body. this should free the frame form the body (except it's tethered by the wires) and the trucks will then no longer hit the end of the body as you try to slide the interior out.
I think you said you covered this part, but also make sure to disconnect the ceiling circuit board for the lights. There is a molex connector in the wire there I believe.
Even after all that, I still had a car that was stubborn in terms of sliding the interior out of the body. I felt like something was binding, and I could not figure out what it was. I was afraid I would break whatever it was if I kept pulling, so I stopped and never did reattach the feature that came unglued in that car.
-Dave