Hi, I've done floor work on my 1966
Globe Trotter and also an Overlander that I no longer have. Of course that doesn't make me an expert.
The outer sheathing overlaps a U shaped aluminum perimeter channel and is riveted to it. The channel itself is bolted to chassis outriggers with the plywood floor positioned between the channel and the chassis. The interior sheathing does not necessarily need to be removed if the area of the repaired sub-floor is small. An extensive replacement that goes from one side of the trailer to the other requires a complete or partial removal of the interior sheathing. I say partial because the bottom can sometimes be lifted up to expose the channel. In the front and rear though the sheating is curved to conform with the corners and so cannot be lifted up from the bottom. It must be removed.
The edges of the replacement plywood sub-floor can sometimes be notched where the perimeter bolts are and slipped (wrestled) into place without removal of the interior sheathing, but again the area should not be too extensive.
An extensive area of replacement will likely also require opening the pan underneath the trailer for access to the bottom side. This is if "elevator" bolts are to be used again. These bolts require a nut underneath and the only way to put the nut on and tighten it is if the pan has been opened.
Some people use self tapping bolts to attach the new sub-floor material to the chassis, but the Aerostress chassis in use in the Sixties and Seventies was not as thick a steel as what was used in the Eighties, and is actually too thin, in my opinion, and doesn't have sufficient thickness of material to provide much hold on the threads of the self tapping bolts. Elevator bolts should be used instead.
I've attached a diagram to provide a visual. You should understand that if the plywood sub-floor required replacement anywhere along the perimeter of the trailer, then likely that was due to rot, and if so then likely the channel and bolts were also compromised to some degree due to rust and galvanic corrosion. The second photo shows that.
Hope this helps.