I saw this old bird in the last days at STN and as others have said it was a dead ringer for a C-9 or VC-9 (if such existed). Photographic evidence showed this to be incorrect and it's heritage was confirmed, BUT it's because it has the partial colour scheme that mimics the VC-25's of the US President and this is what has caused it to be suspected of being ex-USAF.
Boeing VC-25 - Wikipedia, the free encyclopedia
SHJ