Just about everyone that uses this spelling of the surname will be a relative of Milton P. Slavens. With a few exceptions, a "Slavens" will be a descendant of the various sons of John and Elizabeth Slavin of Highland County, Virginia. Six sons left Virginia late in the 18th century to become pioneer residents of Ohio, Kentucky, and Tennessee, and on to Indiana and Missouri, and eventually across the country. A seventh son remained in Virginia/West Virginia, and his descendants usually use the "Slaven" spelling. Over the years some families have modified the spelling so it's not infallible, but I'll bet it works at least 80 percent of the time.
The Reuben you mentioned-- many Slavens families had a Reuben each generation back in the 19th century-- was a brother of Samuel Slavens, one of the seven soldiers hung as spies for stealing the train the General in the Civil War. The group of soldiers have become known as Andrews' Raiders, the episode known as "The Great Locomotive Chase" and Google searches will come up with lots of information.
There's a lot of information on the Slavens clan in general-- and a lot on other unrelated families with other variations of the surname-- on my website at http://www.slavens.net/