A bathtub in a house has a couple of major advantages over what is generously called a "bathtub" in a recreational vehicle. Most obviously, it is large enough to take a bath in, even for someone who is 6 feet tall and a bit overweight. It is in a room that is large enough to dry your back in without hitting the walls. And it was carefully installed at the time the house was built to be tipped ever so slightly forward toward the drain, so that the water in the tub from a bath or shower goes right down the drain with no artificial encouragement. As you can probably guess from my tone, this is not the case with RV bathtubs.
Since I don't have any contacts inside the secretive and mysterious world of RV manufacturers, I can't say for sure, but I suspect that there isn't a lot of worry about the positioning of the tub in a travel trailer or motor home. It goes into the spot designed for it, is securely installed, and that's it. Maybe it's designed with a tilt from one end to the drain, maybe it isn't. All I know is that I have discovered that "level" is mostly a state of mind, a function of where exactly the levelness is being measured, how it's being measured, and whether the tub drains correctly.
I have memories of spending an hour or so getting our old 30 foot camper as level, on average, as possible. I say on average because there was a built-in bubble level on the tongue that I could never quite figure out but that seemed to get me in the ballpark. Then there was the little square plastic bubble level that I could place on various inside surfaces like counter-tops, floors, ledges, and shelves, only to get different readings in every place. And of course there was the fun of going out and making small adjustments to get the average level closer to being just right.
But until it was time to take a shower (no, never a bath - my knees would hit my chin if I tried), I didn't really know if the most important aspect of leveling had been accomplished - whether the water would drain right out of the tub, or if the aid of a squeegee was required to push it along. Of course, by then it was too late and I was not about to go make adjustments if they were needed, especially since I had just gotten cleaned up from the leveling ordeal in the first place. But I know that in my mind, "level" had nothing to do with what the bubble showed - it meant that the water drained from the tub on its own.
Now we have a motor home, and again, I don't know how the makers think about these fine points. Is the tub designed to be tilted when the rest of the RV is level? What if the tilt towards the drain - the inside of the motor home in our case - is too much for the waste water tanks to empty completely? Did someone think about these things when they were putting Joey together, or do I just think too much? I don't know for sure, but I do know one thing: we have to use a squeegee to get the last half-inch or so of water out of the tub, but the bubbles all say level. Life is full of uncertainties, isn't it?