It sounds like you are not using an attorney to represent you in this transaction. Although it is a Realtor's job to help you through every step of the home selling/buying process, issues like the one you are having are best handled by an attorney. I hope everything works out for you.
Read the purchase agreement. In most cases it states that the homeowner needs to fulfill certain obligations. The insurance portion is normally tied to the underwriting of the loan. There is a significant difference between not being able to obtain insurance and the relative cost of a policy.
I agree, check with a real estate attorney, however on the face of your question I can tell you that insurance is the LAST thing that some people take into account, and sometimes it only occurs at the time of funding the loan if the underwriter has a question.
You need to discuss this with your real estate attorney who is handling the sale of your mobile home. If you do not have one, the Illinois State Bar should be able to point you in the right direction. Sorry to be so vague, but as a Realtor I can't offer legal advice.