No leveling here.
Installed 3/4" birch plywood plywood floor with a thin closed cell foam underpayment. The underlayment is available at Home Depot and the like, I put it in to keep the floor from squeeking. Spray glued to the plywood. The plywood was primed and painted with porch paint, a little more durable than exterior paint. My second seat mounts are bolted through the plywood. That's the only attachment.
On top of that I layed some of the foam flooring available in puzzle-piece squares. Makes in a bit quieter and nice on the knees.
Finally I installed, cut to fit, some door mats, the type you walk on as you enter many businesses ect. Mine are Anderson Water Hog mats. They take a beating, hold all the crud in their grooved surface, don't leak through and remove for cleaning. You can spray wash them, hang um' to dry, throw them back in and you're ready to go again for next weekend. Check them out?