Are you sure it was the tower and not the center (ARTCC)? The towers have no need for remote sites, as they are only dealing with traffic in and around the airport, but the approach controls and centers have tons of remote sites. Very likely you were hearing Cleveland Center and Minneapolis Center.
Map with FAA ARTCC RCAG sites
A very nice accurate map, but there are some frequency errors and some of the site locations are slightly off
A very nice map & credit to whoever created this RCAG & frequency locator