If your coverage is continuous, you shouldn't have any problems. If there's a gap, Crohn's might not be covered. The way my dad tells it to me, even if you've had a gap in coverage and they claim preexisting condition, after you've been insured for a year it can't be called that anymore and they have to cover it. A year is a long time though, so if your coverage does end up having a gap and they stick you with the preexisting condition, you should try looking into some of the assistance programs offered by the company that makes Remicade, and some hospitals even have similar programs.
20, Male, diagnosed over Christmas Break of my Junior year in High School. BEST CHRISTMAS EVAR LOL