A map.rwm file is present in the data/world/maps/base folder when you install Rome or Medieval II - Total War. One will be in the bi/data/world/maps/base if you have Barbarian Invasion installed. The map.rwm acts a little like a saved game file containing compiled information from a variety of map tga's and text files.
If you wish to make changes to the campaign map you need to 'delete' the map.rwm file (just re-name the file to xmap.rwm or similar so game doesn't find it and you still have copy). To make changes affecting coast-line you will also need to delete (or preferably re-name) the map_heights.hgt file.
Re-generating the map.rwm
When you start a new campaign after deleting or re-naming the original map.rwm a new one will be created for you. The new map.rwm is created after you select a faction to play campaign as and select arrow to load campaign, it will take longer to go through screen change and loading bar than normal while the new file is being created.
If your game crashes when the new map.rwm should be being created (and it is not created) then you have some error in the modifications you have made. In RTW that can include having too much sea area (the so called Landmass Crash) - having rivers incorrectly drawn in map_features.tga, some gross discrepancies between map_regions.tga and descr_regions.txt and other causes..... Most of the errors that result in a failure to produce a new map.rwm do not give a -show_err message but it is always worth having that on your shortcut anyway.
Note that the map_heights.hgt file does not re-generate and is not strictly required for most game versions.
Where the map.rwm is located
If the map files, map_regions.tga, map_heights.tga etc and descr_regions.txt are located in the base folder then the current map.rwm will also be in that folder.
If you copy the map files in the base folder into your imperial_campaign / barbarian_campaign / or provincial_campaign* folder then the new map.rwm will be created there. If you have multiple provincial_campaign folders, which have all the map files in them, then you will have different map.rwm's in each of the folders. (*at time of writing provincial_campaign function not available in M2TW)
For a substantially altered map you may need to have the new map files and new map.rwm in the campaign folder, and retain the original map.rwm in the base folder. The map.rwm in the base folder may be needed for custom battle locations to be found. (last paragraph needs verification)