If you have to segment the repair make sure you put a scab patch under all of the seams. Overlap each side of the seam by at least 4". Saturate both sides of the mating surface with glue - it will wick up into the plywood. While the glue is still wet install enough screws to facilitate a tight 5 spot pattern on both sides of the repair. This will bond everything together and make the repair stronger than the original.
I found an almost perfect match for the floor on the '78 at Home Depot - see posts #20 and #27 of my Sovereign
rebuild. It was an 8 ply finish plywood with exactly 1/2" thickness. I would think that the 8 ply is much stronger than the most 1/2" 3 ply commonly found today.
Make sure all of the frame welding and repair is accomplished prior to repairing the floor. While you are under there, please strongly think about adding a 20 amp electrical service. I use the second service for the second AC, an additional electric heater in the winter, and with both the electric element and burner going in the water heater I have almost unlimited hot water. None of the above uses were seriously contemplated when I did the rear floor repair - I was still going to flip the trailer at that time.
While you are in the back bumper storage area, you may consider installing a new electric cable. I have been in a couple of instances where I was measuring low voltage in a park environment. #10 wire gets marginal when a total length of 100 feet is approached with even a 20 amp load. I have witnessed 95 volts in the interior of the trailer with a 200' 10 guage feed length. I carry 75' of #6 umbilical line for the primary electric feed and 50' of #10 cable for the secondary feed. Just the electric cables fill up the rear storage area. Try to minimize the number of electrical connections in the circuit. Each connection, whether permanent or plug in, adds to the overall voltage loss.