At age 18 you can do as you choose. You do NOT have to notify your parents of your choice or your actions, just bide your time and play their game while your enlistment process winds its way to a conclusion.
At the time you have to leave for Basic you either tell them or you don't. You have no obligation to them at that point.
I agree...why the HELL would you want to send money to reward your parents for doing a lousy job? I had genuinely wonderful parents and stood by them through their final years, but they were loving, caring people who supported my decisions and assisted me when I became a single father through divorce.
My father gave me some excellent advice that a lot of people, including your parents, do not understand or practice. He told me, "When your children become adults you can do one of two things: You can try to remain a parent and tell them what to do in life, in which case they will come to resent and hate you; or you can release control and try to become a trusted friend, in which case you will stay close for life."
Just like with my father and me, both of my grown sons know that I am not going to "tell" them what to do and only offer an unsolicited opinion if I feel they are genuinely at-risk. However, if they want to ask my opinion for guidance I will give them the best possible information I have based on my knowledge and experience...and they are free to ignore it at their own risk without offending me in the least.
Bottom line is to do what you feel is your path in life and do it in a way that does not disrupt your life for the last few months you are at home.
Good luck.