There's a lot of suspicion with the use of RTV in place of gaskets and rightly so if you've seen some of the messes out there from using it incorrectly.
I use it all the time and don't have problems, that's because I put a bit of thought and care into it.
How I do it is -
clean both mating surfaces,
apply a bead of RTV to the movable surface (the diff cover)
apply a thin coat of oil to the stationary surface
when the bead is "off" but before it sets, apply the cover to the diff and press gently making the bead form a gasket.
screw the bolts in just enough to hold the cover on without squeezing the RTV out - this step is very important.
when the RTV is set torque the bolts up.
You now have a silicone gasket that is formed in place and should come off the diff ok.