I'm wondering if you read my post before yours...Moncton won't be hosting 2015. They're only a placeholder.
As for American teams hosting:
Spokane in 1998
Seattle in 1992
Portland in 1986 and 1983
Last five hosts by League:
WHL: Saskatoon, Brandon, Vancouver, Kelowna, Regina (2013-2001)
OHL: London, Mississauga, Kitchener, London, Guelph (2014-2002)
QMJHL: Shawinigan, Rimouski, Moncton, Quebec, Halifax (2012-2000)
Chicoutimi have never hosted the Memorial Cup.