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The floor goes between the c-channel and the frame. One way or another, if you are replacing a large portion of the floor (more than just a small patch) you will need to access this c-channel - which is most easily done by removing an interior skin. In order to replace the floor properly, you will have to remove the lower interior skin to access the bottom c-channel track to attach the shell to the frame through the new floor. I guess an another alternative would be to remove the exterior panel...but that seems a bit drastic and could compromise the exterior with regards to leaks.
Shari
