HR Law happens to be my specialty!
You have a right to privacy and should not disclose anything that they do not specifically ask. If they specifically ask about Crohn's, which I doubt they will, they you should be honest because you pre-employment physical is part of your hiring process. If you lie and they find out you can be legally terminated immediately. But be sure to only answer their specifc question. There is no need to elaborate
Most likely they will ask you if there is anything that would prevent you from doing you job. Answer carefully. If you have a history of leaving jobs because of this illness you must disclose the crohn's. You answer should be diplomatic but truthful. "Not at this time. I do have crohn's disease but do not have any current limitations that would impede my ability to work as a result of the disease." Make it no big deal and they will too. If you do not have this history than answer no.
Lastly, you have already been offered the job. At this point you are basically an employee and are afforded most of the same rights. There is a verbal contract in place. They can not decide to not offer you the job based on the fact that you have crohn's unless they can prove that it would prohibit you from performing your job duties. If they did take back their employment offer you would have grounds for a lawsuit and would be able to apply and win a unemployment claim.
Trust me! I have seen it done many times.