I’m not an expert in image processing or anything but I believe you need an intermediate step in the method described above by xy mosian.
If you import the .tiff into inkscape then save, you’ll end up with an svg file which contains a bitmap i.e. the save function doesn’t convert the .tiff to a vector. To do this, once your .tiff is imported go to Path > Trace Bitmap. There are loads of settings in the dialog box but for a simple outline for cnc work ‘edge detection’ will probably be appropriate. This should give you a vector, then you can delete the old raster image from the file and save.
Another thing to note is, when exporting the 2d outline from sketchup, set the camera to ‘parallel projection’ and use the ‘views’ toolbar to set the camera view directly above whatever outline you’ve drawn, and you will probably need to rescale the vector image in inkscape to get it to the correct size.
If you don’t want to save as svg regularly the easiest thing to do is to install the free trial of sketchup pro alongside the free version. Draw your items in the free version, then when you’re done, open them in the pro version, export and quit. You get something like 420 minutes of pro trial so it can last a while if you use it like that.