Maybe if you can pitch in to cover the difference in fuel you can still go. I know that gas is expensive but if your mom was going to pay for the rest of the trip I would say volunteer to cover the cost of gas from Savannah and back.
I have no idea what your financial situation is but you are looking at about a 600 mile round trip. The average price of a gallon diesel fuel in FL right now is about $4.60. A 2000 Ford F-150 with a diesel engine can get about 18-20 miles per gallon on the freeway. If you go with the low number, that is about 33.3 gallons to get from Savannah to Orlando and back. You are looking at about $153 in gas. I know I would be willing to pony up $153 for a trip to WDW any day.
Hope you can work it out though.