Not the greatest photos in the world, but these show the space under the back wall and the floor at the back of my 1964
trailer, without any of the belly skin in place:
Yours may not be exactly the same, of course, and you won't have the cleats that I have there holding in a floor patch unless you also repair the floor.
Anyway, removing the belly skin in that section was quite easy for me, as it was a seperate rectangular piece there. Under the back wall of the trailer was a flat piece of steel crosswise between the frame rails, with the major connection between the frame rails being the back bumper. Now, in mine that was also where the black tank was located, and if that's the case for you too, you'll need to put the grey tank somewhere else.
I plan on putting grey & fresh water tanks much further forward, but again without removing the shell, just cutting open the belly skin.