Extend Mac display to Windows 10
This looks like it will work. It's not free, but I haven't seen anything else that works. It uses a VNC client for the "other" computer, but it says any VNC client should work so you don't have to buy theirs. You only have to buy the Mac program. There are plenty of free VNC clients for Windows. There's a free trial so you can download it and see if it works. 

https://www.screenrecycler.com/ScreenRecycler.html