Ok... after long thinking I managed to solve the problem without success.
First I tried to squeeze the RTV through a small tube: mission impossible!
My last chance was to drill out a good number of rivets to open the elevator and put the #@@#!!! RTV where it was supposed to be, but I ended up with some slightly enlarged/elongated holes on the skin. I have 4 of them in a row along the spar and I don't think that using oops rivets there is a good idea.
Perhaps it would be better to leave everything as it was!! (Phil, you were right!!)
My final decision: I'll build a new elevator!!

(I hope I can save at least the understructure!!)