As long as you have any leaks fixed, and fixing them are about the same as fixing them on an aluminum boat, you can use any construction grade CDX plywood, I am sure you will be either covering with tile or carpet, don't try ceramic in a rig that old, they flex to much, we always cover with a decent quality indoor outdoor or a Burber that can be glued down. If you are going to do padded carpet, then you will have to put tack strips around the edge and use a stretcher to put it in correctly.
Does this unit have an aluminum belly pan on it? If so, before you put the new floor in make sure all of your pluming is working correctly before laying the floor down, it is much easier to work on the plumbing from above, than trying to work on a unit with a belly pan. If you have to cut a belly pan, you will never get it sealed correctly again. I would imagine, that rig probably has copper plumbing in it, be sure and air test the system, pick up a water regulator and set at 45 pounds, then a screw in air valve and fill the system up with air to make sure there is no little pin hole leaks in the system. Double check your holding tanks, all of them, black, gray and fresh, many of them older rigs had galvanized fresh tanks, that after time start leaching solder into the water, which back then had lead in it, if there is any signs of seepage on the fresh water tank, replace it with a new poly fresh water tank.
Helping my buddy, we have done some extensive restores on this type of trailer and if you get it right, they are worth a fortune and will last a lifetime, if it is done wrong, they are worth nothing and become excess baggage, Do it right and you will have something to be proud of, that can be handed down for generations.