Do Americans ever use the word 'loo' to refer to the toilet/bathroom?
No. Well, sometimes if I'm pretending to have a British accent.
Not unless you're a transplant. Bathroom and restroom are the two most common. Some parts of Wisconsin, Michigan, Minnesota, and the upper East Coast will use the term washroom, too.
Little Big Man
No, they use bathroom.
They Pelted Us With Rocks And Garbage
No Not unless they're one of those insufferable people who watch the Eastenders on PBS and want everyone to now they do.
The use 'loo' to refer to Waterloo.
US uses bathroom, powder room, john or toilet. out in the country it might be outhouse. In a public place like a restaurant it wound be men's room or ladies room.