I put a dinning-room table place mat on the floor of the shower. It fit perfect in the recessed area, covering the shower drain, too. It's water proof, easy to clean, dries quickly, has a scratch resistant bottom, adds a little color and is easy to stand on with bare feet. I didn't want little squares/holes for the dirt to get past (now the dirt gathers at the edge of the mat, I guess).
I looked for one of those rubber bath tub mats that have the little suction cups on the bottom, but I haven't found one, yet. I figured one of those wouldn't slip and would retain what little dirt we leave in the bathroom from the bottom of our socks/bare feet. Typically, shoes don't go in the bathroom, but the camper is still brand new. That attitude may change in 5 or 6 years. Perhaps sooner, if I get to mountain bike as much as I should.