Are you using your form for Data entry or to display information already held.
Either way i'm guessing your form based on a query. In the query link the tables via the RoomID fields. This will allow you to select the 'roomname' as a field, you can then make'roomname' the data source of one of you unbound text boxes.
Alternatively use DLOOKUP
=DLookUp("[roomname]","tblRoomNames","[roomID] = " & [Room])
All the best
Stephen