I appreciate your post and you may be able to find him but I have looked in a lot of places.
Joseph B. Johnson, Pvt., 15th Maine Inf., Co. E.Died of Typhoid Fever at Georgetown, SC July 7, 1865.
First of all be aware there were 500+ Joseph Johnsons that served.The one listed at Beaufort is the wrong one.He is not in the Nat. Park Service Grave Locator.I have also checked all the Johnsons at Beaufort. Find a grave, Etc.
It took some doing but I was able to identify 6 of the seven men of the 15th. that died at Georgetown in July 1865 and i'm sure I can i.d. the 7th.There were 10+others that apparently died in hospitals "someplace".Of the 6 I i.d.ed only 2 show up in the Nat. Grave system.
I would sure like to look at Section 20 at Beaufort around sites 1821 & 1824 but that's impractical.
I suspect that the cedar planks used at Georgetown as grave markers may have been vandalized or perhaps used for firewood before the Army came to remove the bodies.The details in The Roll of Honor might answer that question.