It depends on whether the disability (which I assume is NOT Social Security Disability) is taxable income or not.
Disability income, if provided by the employer, is taxable income. If your daughter's disability is taxable, then your daughter CAN claim her son on the tax return. Since the disability income replaces the earned income your daughter would have received from her employer, she ALSO qualifies for the Earned Income Credit.
Same logic applies for you. However, to claim your grandchild, you must have provided over half of his total support for 2007.
As for the property tax, it is an ITEMIZED deduction, which is deductible if all of your itemized deductions exceeds your standard deduction, which is $5,350 of you file as a SINGLE person or $7,850 for Head of Household (a status you qualify for if your grandchild is living with you and your daughter in the trailer).
If this seems confusing to you, you may want to consider getting professional tax help.