If it is the large ball base you will need access to the backside. heck even if it isn't you will need access to the backside.
The screw heads that are visible have normal nuts on the inside holding it in place. The only real option is to cut a hole in the inner skin and do what you need to, then patch over the hole. That is what all of the service manuals I have read from Airstream say to do. They also say the same thing to install.
If you are willing to take things apart you could remove the section of inner skin that is on the wall inside of the antenna. Do what is necessary, and reinstall it and all the other stuff that had to come out.
I have attached a picture of what the inside wall looks like with the inner panel off. In the lower right corner, to the left of the window, you can see the back of the antenna, it is the small round thing with no insulation around it.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato