You've actually got two issues here- first, you shouldn't rely on the Comm
Manager to shut off data when abroad.  It isn't a "hard" toggle that shuts
down data until you turn it back on, it simply terminates the current data
session.  ANY data using program: Google Maps, email, web browsers, etc. can
and will turn it back on when they next need it.

If your device has no "turn off data when roaming" function, the safest
thing to do when abroad is to switch your chosen "networks" in
Start/Settings/Connections(Tab)/Connections'Advanced(Tab)/Select Networks to
one without any celular data connections.  As added insurance, I "munge" my
cellular data settings- intentionally misspell the cellular APN so even if
the odd program overrides all device data settings, it will get an error
instead of connect.

Having said all that, now I'll REALLY disappoint you!

Unless Google Maps has changed radically in the last several months since
I've used it, it requires a constant data connection.  Trying to force it to
run without one (used to at least) generate an error message saying the data
connection was lost and asking you if it you wanted to retry.  This nag
message would pop up often enough to make the program unusable.  Instead,
you might try Microsoft's Bing for Mobile.  It's similar to Google Maps, but
it caches map data for use if your connection is lost.  I used it last
winter on a trip to Mexico without cellular data by connecting to hotel
WiFi, opening my location, and scrolling around the screen in all directions
at a few different zoom levels to load as much map data into the cache as
possible, and was able to use the program without a data connection.  (Of
course functionality without connectivity was severely limited- you can't
get directions or search for nearby businesses, etc.- it just functioned as
a "moving map" with my location shown on it.  Since I really just wanted it
to find my way back to the hotel if lost, it was good enough.)