Here's what you need to do to get the Document tabs in v18 not to be displayed when you have just one window (like v17).
Close all files, open one file, use Zoom Toggle (Window>Zoom Toggle). The Document tabs should be gone now. Now close the file so you have no windows open any more. SlickEdit will remember this (it's an undocumented trick).
After performing these steps, when you have just one window, the Document tabs won't be displayed! Now v18 will be operate a lot like v17 with regard to screen real estate.
This is the way I work but I don't use the File Tabs tool window. I live on tag navigation, smart open, and directory aliases for opening files. Once in a while I use list-buffers to reopen a buffer. It performs the same function as File Tabs but doesn't take any screen real estate. Since I open tons of files, this works better for me than the File Tabs.