Howdy CH, another great question-
A curious verbal description that I have also wondered about many times. Of course trailers aren’t “curved” they’re straight (in plan), virtually without exception, as they are designed and built within the limits of what is legally allowed to be transported on the public highway. They’re truly modular, factory-built housing, pulled to site on its own set of wheels. So yeah, why does AP say “curved”?
Could it be that AP was referring to the profile of many trailers, that while the floor plan is by necessity flat and straight the roofs are sometimes curved. This would be true for a certain class of travel trailer, especially those of the Airstream manufacturer. These are certainly curved in profile, and maybe this is what she’s referring to. But here’s the problem with this reference, these are typically exclusively “travel trailers” not mobile homes. As travel trailers they are designed to be towed behind a vehicle for recreation not parked as a semi-permanent home. They are generally quite expensive, really the “Cadillac” of travel trailers, not at all whut poor Ennis could afford.
Typical AirStream Travel trailer

The more typical mobile home type trailer or even more middle class camper that we see Ennis in at the end of the movie is really a quite rectilinear affair, very boxlike, not curved at all. Small camping type trailers did have some curves, to make them slightly more aerodynamic on the highway, but generally only at the front and back. Not “down the length”. This could be one of the few, very few, instances where AP misses the mark with her word picture of the scene. While it sounds quite convincing, it may not actually be what one would find in lonely rural Wyoming, inhabited by an almost destitute cowboy.
Standard camping trailer

As a side note – urinating in the sink – even the smallest post-war travel trailers typically had toilet facilities, this is what distinguished them from campers on the back of a pickup. Urinating in the sink is more of a convenience for a single man living in a trailer, it’s easy, handy, and uses little stored water compared to a typical toilet. It bespeaks of Ennis’s basic work-a-day character and modus operandi, the sink can function as a countertop urinal as long as you have the "equipment at hand".
