I looked around quite a bit for vents that weren't 14" (I had a 9" vent), and I don't remember seeing any 12" ones, just FYI.
14" is the standard now, it seems, in RV vents.
However, there is one other thing you might check: Boat vents, which are called "hatches." They come in a number of sizes. They are higher quality than RV vents but will be more expensive.
Bomar is one common brand. The least expensive, least "seaworthy" one would still be overkill compared to an RV hatch (meaning you could go with a "cheap" one). There are other brands too - you could check someplace like www.defender.com
(Defender's catalog is less slick but their prices are more agreeable).
Or... like you say, get out the pencil and saw and go 14"