The campaign works in one of two ways - either it uses RTX files (which are texture files that use a pre-coloured DDS image) or it uses the RSH files. IIRC Dawn of War used the RSH, but all of the subsequence games use the RTX files (because there are multiple RTX files - normally eight - and so they can mix groups from the same race on a single map).
The files you want will be in the "art/ebps/races/space_marines/texture_share" folder of one of the SGA files (so you'll need something like the SGA Explorer). As long as the files you edit are in the right place (which most SGA apps default to) then the game should automatically load your changed versions.
In terms of finding out whether it is the RSH or the RTX and which RTX you need to edit, I'd recommend extracting the RSH and all of the RTX files, then extracting the textures from the files. Make an obvious mark on the RSH and on the Blood Ravens RTX (but make it a different mark to the one on the RSH), recompile them and run the game. You should be able to tell which it is using based on the mark you can see.
Telboy made a mod to put the Lamenters in the Dawn of War and Winter Assault campaigns. I think DoW used the RSH and Winter Assault used the RTX. You might be able to get some hints from that