The Arlington Plantation was in Northampton Co., VA & was located on the west coast of the Eastern Shore. It is the site of the small graveyard called "The Custis Tombs." This graveyard is now in a residential neighborhood, but you should be able to park on the street & walk up to it. This side of the family was most famous because Daniel Custis married Martha Dandridge, who later married George Washington.
Ravenswood Plantation was on the east coast of Accomack Co., VA (one county to the north of Northampton), and was the home of a different branch of the Custis family.
I am descended from this second branch & can share data via my direct email below.