The key here is, what are the specs of the cam. Generally what most call an 'RV' cam is really just the non-HP cam which carries a valve lift of around .435" and 256°/260° duration, about 204°-206° or so @ .050". Lobe separation angle was wide on those factory cams too, the common replacement carries a 113° LSA (increases vacuum). Your carb is about the size of what would have come on a 383. To add some perspective I was getting 15" in a 440 with failing rings, the old comp XE274 (110° LSA cam w/ 60° of overlap and an 850 Demon carb, with other performance mods) So, in my opinion you should have a few more inches of vacuum happening BUT 15" is okay, provided it runs and idles well? It could very well be you've just got some carbon buildup in there. The main thing is, is the gauge fairly steady or does the needle bounce around a bit; if so, how much. Run a can of sea-foam through the carb, or some say to even just put a hose on the carb's vacuum nipple and let it suck a glass of water through it and re-check. I've found that cleaning the buildup out can increase vacuum a few points. Needle fluctuations can indicate sealing and/or valve issues. Important note: Your state of tune will also affect vacuum so make sure your carb and timing adjustments are good before getting too worried..
