If the USL team you are talking about in Charlotte is the Eagles, nobody goes to it because it is in a crapy high school stadium and they dont play anybody. If you construct a nice stadium that is small (but easy to expand) say the Battery's stadium that would help. Plus alot more people would come to see MLS players than USL players.
I dont know about Greenville I mean ya there is alot of soccer in Greenville but is is not that big of a city that is the reason I left the Charleston area off the list.