Again, it may be too distant to know for sure. From my readings it is most probable the legions had permanent winter camps which they would return to at the end of "fighting season". It would seem possible, if not probable, these winter quarters were constructed of non-portable materials.
IMHO, regardless of what exactly the reverse represents (city gate, camp gate, or watch tower) you original thesis should not be affected - all three possibilities are advertising safety and security. A very fascinating idea about the human mindset in times of perceived crisis, both ancient and modern!
Though it does not talk about campgates, there is a book by Cullen Murphy that has a chapter called "The Borders" - Are We Rome? The Fall of an Empire and the Fate of America
I've always thought the reverse was a city gate. More importantly, as mentioned above, these gates (from either a camp or city) or signal towers represent security and safety, probably from the frequent and destructive barbarian invasions. Put my vote down for city gates. The picture above is the Porta Nigra, the large Ancient Roman city gate in Trier, Germany.
Hmm... Coincidently, I picked a modern Campgate out of a pile of junk world coins this weekend! 1919, the year after WWI ends... In case someone doesn't make the connection, depicted on the reverse is the same Porta Nigra pictured a few posts above.
Also see here. Now the Porta Nigra was most probably a "prestige" building. Of course it served as a city gate too, but according to most experts, Marcus Aurelius had it built for that purpose (representation) except it never got fully completed due to some lack of funds ... Christian