You can get pressure treated but I would think just
regular garden variety 3/4" A/C would work just fine.

I have an inexpensive utility trailer I bought about 17 years ago. At the
time, I just bolted on standard 3/4" AC plywood. 17 years later, and it
still looks and feels solid. Very worn and abused, but solid.

On the other hand, I had built some sides out of 1/2" CDX plywood and
2x4's. Because of the trailer size and the way the sides were constructed,
I had to take the sides off the trailer to carry plywood and other sheet
goods. So, the sides sat out on the ground a lot. It only took about 5
years for them to rot away to practically nothing.

So, I rebuilt the sides with PT lumber and 1/2" PT plywood. I also built
them in a way that let me permanently attach them and still be able to
carry sheetgoods. Another 12 years or so, and they still look like the day
I built them, except for the scratches and dings from carrying things.

The final and ultimate is PT 2x6 bolted on... they will
be there when the trailer is toast.

That's my plan when my 3/4" AC floor finally goes.

Anthony