Well, you made a commitment to them for a specified period of time to get a discounted rate. Based on the time you were there, you could offer to "convert" your contract to the weekly, monthly or whatever rate. That way you're not cheating the park and all would be good...that said, I'd use that as my second choice...my first choice would be to simply explain the circumstances and hope they understand and let you out of your obligation...
Good luck...and I pray for a speedy recovery for your loved one....Dennis