Growing up in the South, I was taught to say "Rest Room" or "Ladies Room". "Toilet" would have been looked upon with horror. "Powder Room" was also used to refer to restaurant ladies' rooms. Euphemisms are big in the South.
Once one has had children, "Potty" becomes the preference and seems to remain even when the children are 40 and have children of their own. However, my genteel Southern upbringing still kicks in when I'm in polite company, so I would never use potty then. BTW, I never heard the term "Wash Room" until I left the South and went to the Chicago area. I've never liked it and don't use it. To my knowledge , "Loo" is never used in the U.S. Of ourse, in more remote areas, we have "Sanikans" and "Outhouses".
Signs on the doors of public restrooms are always interesting too. Sometimes, instead of "Women" and "Men", they say "Ladies" and "Gents". And in waterfront dives, there is the inevitable "Buoys" and "Gulls".
Jude
