That looks bad! Some one did not have love for Jellybean like you do.
If it were me I would patch it for now and save the pennies to replace the antenna and do the job once. The smaller spot is factory. There should be a 2x2 plate with 4 screws under all that sealant. This is where the antenna down-lead entered the coach originally.
If you replace it with a bat wing, the lead will go thru the base plate. The base plate should cover the horrid hole that you see now. I know you love that antenna, but all the parts are NLA.
Once you open up the spot to fix the leak right you are going to want to finish the job. I have some excess aluminum skin from a mutual friend If you can let me now a size I can get you a patch to cover where you take out the old antenna.
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