“The Right Stuff” is a neutral-cure silicone RTV (doesn’t liberate acetic acid and smell like vinegar, as some silicone RTVs do) and is very similar to Permatex’ “Ultra” line of sealants/gasket makers (blue, black, grey, etc). Permatex probably recommends against using such cured sealants around gas because they are readily swollen (but not dissolved) by such solvents and once swollen loose much of there mechanical strength (the mechanical strength is modest even in the virgin state). In your application you are not so concerned with mechanical strength as you are with having a compound present in the thread to prevent air leakage. The diffusion path from the IN port back through the nipple thread is relatively long and I can’t envision any scenario where fuel penetration could remove any of the sealant. Fuel could swell the sealant and cause it to expand, but that’s all that can happen. So even in the swollen state it would function as a satisfactory seal. I’m sure with use the sealant goes through perpetual cycles of being slightly swollen when liquid fuel is present to being totally free of fuel as evaporation and heat cause fuel loss.
In spite of the swelling potential I suspect you could use the product successfully in your application. You could also try Permatex’ Hylomar which is also non-hardening and additionally alleged to be resistant to all automotive fluids including gas and gas/alcohol blends.