An f-stop is double or half the amount of light. An f-stop over is twice as bright, and f-stop under is half as bright. The Zone system is based on f-stops, so Zone IV is half as bright as Zone V, which is half as bright as Zone VI. Zone V is middle gray, the same as an 18% reflectance gray card, which some quote at R=G=B 117 for gamma 2.2.
However ... regarding RGB, apparently Lee Varis sets the R=G=B values at this, for zones 0 to 100:
Now if you're a really anal Zone guy, you might want to do your own densitometer/paper testing as gamma (among other things) are an issue. Your mileage may vary.