The only "single drop" in relation to campgrounds I've found so far is related to water falls near them that drop straight down with no intervening steps. For example, Taughannock Falls State Park in NY's Finger Lakes Region is said to have the highest single drop waterfall east of the Rocky Mountains...
TC&Z wrote: Never seen this before, what does it mean when a campground describes a site as "single drop" ?
Drop camping is a backpacking term where you are dropped off by an outfitter at a trailhead. I'm guessing if it's a campground for tent campers then it may be a single site for one person or tent. Do you know for a fact the campground has RV spots? If so, it may also have spots for tent campers.