291 or 291 fast cure are the ones commonly used to bed hardware on boats.
Now for a controversial comment:
Running a thick bead around fittings does very little - in my opinion - to seal anything. You will not see this approach taken on a boat. It is the sealant under the flange of piece you are fastening that is doing the sealing. This is protected from the weather and UV by the flange, and compressed under it by the fasteners. Now I realize that I am at odds with nearly the entire RV industry which believes slathering sealant all over the outside of everything is the solution. You will never see this done on a properly built boat, which sees a lot more water, and the consequences of leaking potentially fatal.
I will note that the brackets of many awnings are very poorly designed, with fasteners bare through the skin and no flange near it to seal. In that case what else can you do but slather? They are designed to leak.