Mansions generate in roofed forest biomes, and are somewhat rare (although not as rare as they are on java). In a large size map there is a good chance that one will generate, although its not guarenteed.
One way to find structures easily is to generate your seed again in creative, and fly around and generate all the terrain. You can identify where points of interest are (temples, mansions, monuments, etc). Then note the coords and go straight there once you've loaded your survival world. The loot is random still, I believe, so that will not be the same, but the locations should match from world to world. Some feel this is cheaty, but, since we get limited everything on console I don't invest time into a seed that doesn't have the things I want.