The plane shown is an EP-3A or B. The ventral trough has a series of various intercept antennas in it for ELINT (typically two steerable cones and two steerable cassegrain antennas). It replaced the EA-3 "Whale" in service (the Smithsonian wanted them back.... ). It is purely a military aircraft used by the USN for electronic reconnissance.

The various RC 135 series aircraft breakdown by speciality into a number of sub variants for things like ballastic missile launch tracking, ELINT, that sort of thing.

The one that does most of the ELINT with the Southern Border are EC 130 Compass Ghost and Compass Call aircraft. These are equipped to monitor civilian radio and television broadcasts along with things like cell phones and that sort of thing. They are used to back the drug war helping monitor various cartel's and gang's chatter. These are readily recognizable by the large radomes on each side of the rear fuselage and the antenna spreader under the tail (this forms an antenna around the tail planes in the form of a large diamond).

Fort Huachuca might also be flying various Guard Rail tactical ELINT aircraft in support of border operations too. These were designed to monitor tactical battlefield radio traffic and identify units and command structures. They are based on a Beechcraft twin turboprop aircraft.

The last aircraft that I saw that piqued my attention as a "spy plane" was a plain white 707 with an "N" number (eg., civilian US aircraft number) in small green letters on it and no other markings. Since it sprouted a number of blades, antennae, and other protrusions it definitely had ELINT written all over it. The crew happened to be eating in the dining facility (this was on Davis Monthan AFB) and were in typical military flight overalls but without rank or unit insignia on their outfits. Now, that looked to me like a spy plane!