CVTank.w3d is not the mammoth, it looks like the BattleMaster.
gxmammoth_a.w3d and gxmammoth_*.w3d are the Mammoth Tank.
First you get the w3d files from Generals and put them into a folder then get the ones from Zero Hour and put them in the same folder and wa-la you have all the w3d files and the Mammoth Tank. It doesn't look like the one in Renegade or C&C, but hey Generals isn't really a C&C so only the name counts as well as the power the Mammoth Tank has.
Hint- just get it from Renegade and remember to drop the polys and other things.
*=what ever happens to appear after