I managed to get my mansion back by looking up my backup file that was made when the world generation changed in 1.13, get in to that world, get to where the mansion should be and then copying those chunks and replacing the ones in my current world. I guess this will only work if you were never in the mansion area when the pre1.13-backup was made.
So if a backup was made on your world it should be in .minecraft/backups (.minecraft is the same folder as the saves-folder is in). Extract/make a copy of that world and load it so that it updates to 1.13 format (no backup needed this time). open for lan with cheats and type /locate Mansion. Wait for the coordinates to be written, go there and make sure all of the mansion is rendered/shown. Exit the world without moving outside of the mansion area and now go to the world folders /region folder. Sort the files by last edited. These files are named by some kind of coordinates with format xx.zz or so. You should be able to see some recently edited files which coordinates differs from the other files (at least if you hadn't been close to the mansion area before 1.13). Copy the recently added files with far coordinates and look for files with the same name in your current worlds /region folder. Delete all the files that appear to have coordinates that are close to (or the same as) the files you copied and then paste the copied files - alternatively you may just paste the copied files without deleting any first and just tell the computer to replace them. This last alternative may cause less a smooth edge towards the mansion area.
SOLLUTION!
I managed to get my mansion back by looking up my backup file that was made when the world generation changed in 1.13, get in to that world, get to where the mansion should be and then copying those chunks and replacing the ones in my current world. I guess this will only work if you were never in the mansion area when the pre1.13-backup was made.
So if a backup was made on your world it should be in .minecraft/backups (.minecraft is the same folder as the saves-folder is in). Extract/make a copy of that world and load it so that it updates to 1.13 format (no backup needed this time). open for lan with cheats and type /locate Mansion. Wait for the coordinates to be written, go there and make sure all of the mansion is rendered/shown. Exit the world without moving outside of the mansion area and now go to the world folders /region folder. Sort the files by last edited. These files are named by some kind of coordinates with format xx.zz or so. You should be able to see some recently edited files which coordinates differs from the other files (at least if you hadn't been close to the mansion area before 1.13). Copy the recently added files with far coordinates and look for files with the same name in your current worlds /region folder. Delete all the files that appear to have coordinates that are close to (or the same as) the files you copied and then paste the copied files - alternatively you may just paste the copied files without deleting any first and just tell the computer to replace them. This last alternative may cause less a smooth edge towards the mansion area.