I took aluminum roof flashing from Home Depot and installed it above tires. You can get it in different widths and cut it to size you need. I installed it with tires on, hindsight I would have removed tires to make it easier. Used short sheet metal screws.
Had blowout with a China bomb tire(installed 5 Goodyear Endurance tires when I got to campground) and the metal protected underside from damage. You can see witness marks where blown tire rubbed against metal but it stayed in place and protected black material underneath.
Things to consider:
1. Width of aluminum flashing to buy.
2. Length of sheet metal screws - install about every 4 to 6 inches. You don't want screws to go through to other side of flooring; I think I used 1/2 inch screws. If you don't put enough screws in and have blowout it will rip aluminum flashing loose.
3. Remove tire to make installation easier.