If you’re going to crack open your floor, you can see what you’ve currently got. It’s probably an OSB type or standard plywood. The OSB looks like big chunks of flat wood glued together. The standard plywood will be composed of multiple layers of thin wood glued on top of each other. You’ll want to match the same height you’ve got, 3/4 or 3/8. I’d stay away from anything that has particles, like particle board or MDF. Anything that’s just particles glued together can swell and warp with any contact to moisture.
If you just replace pieces of your floor, be sure to get a leveler to fill in the cracks afterward so your new floor can lay evening on top. You can get a leveler that you spackle on yourself, or a more liquidy one that will level it’s self. Be sure you’re parked on an even surface before doing that though.
Best of luck with your project!! 😀 Let us know how it goes.