You've probably read this from other threads before but...
I find that when we level the RV and then open the slide, it puts extra weight on the slide side which is on the same side as the tub. and keeps the water in the tub.
If I am too level or worst, a little low on the passenger side of the RV, the water leaks out a bit from the side of the tub onto the floor. I may get a shower splash guard or cut a piece of clear acrylic and seal it with clear silicone.