Dicor vs everything else: I have used Dicor and that is what most RV manufactures use. One advantage of Dicor is that it sticks to Dicor. If you use something else you will need to clean 100% of the old caulk. Not much will stuck to silicone based caulks so they need to be cleaned 100% before recaulking. I think it's a choice, not a clear winner.
Eternabond: I have not used Alpha tape so I don't have any comparison. I used Eternabond on the roof of my old Hybrid and it stayed sealed for 10 years (sold it 6 years ago and would guess it is still working).