crontab is for time specific jobs, it is indeed the RC<x>.d directories you are interested in, just put a script that runs the job in the relavant run level diretory called S<xx><JobName> where the xx = a higher number than anything else, so it runs last, but must be less than 100, and the JobName can be anything you like, S = start and K = kill, dont forget to make it executable (and to test it unless you have . in the path you will need to type ./S<xx><JobName> to run it)
Hope this helps if you dont get web thing to work.