@Brus- "loo" is often seen around here (eastern Kansas/KC Metro) as a conscious effort by the speaker to affect Britishness or appear eccentric. In my experience, it's referred to pretty evenly as either "restroom" or "bathroom" in polite or professional company; never "Water Closet" or "WC". We get enough culture from The Isles to know what it means, though.
Among friends or otherwise engaged in leisure, it varies widely- "shitter", "toilet", perhaps a terse "hold on, gotta pee" or something making light of it, such as "I am going to wreck your crapper" (a personal favorite) . I've taken to ironically referring to it as a "terlit", aping a Midwestern rural accent. The more alcohol that passes my lips, the more I find the irony wanes.