I have a plum tree that is about 20 feet high. It has about 4-5 branches coming of the main trunk, There is about a 7 inch deep cavity in that area. I also noticed there is a hole in a pruned off branch. Is there something to fill this cavity and hole? Should i try to drill through it so that it drains? I noticed there seem to be a lot of sap coming out of the trunk area. I would really hate to lose this tree.

Do not drill drain holes and do not fill cavities. The decay may cause structural failure problems if it is extensive but should not pose a health threat on its own.