It has not been confirmed to my knowledge that it was Casey who stole the gas. You can go pretty far on a tank of gas and Orlando is on the ocean, I posted earlier if anyone knew whether Casey might have gotten access to a boat, but I never got an answer.
I believe the woods around her parents house have not been searched because LE doesn't believe the baby is burried there, probably based on some kind of evidence or indication of Casey's timeline.
Plus, let's be honest, the child was missing for 31 days in Orlando in the summer, by the time the 911 call was made, most likely the body was already too decomposed to make much differerence in the case.
It does seem to make sense to search possibly around where the car was found, but again, 31 days is a long time, nobody can even pin down the most likely week the child really disappeared...let alone where Casey was when this occured.
It makes sense to me that police are trying to retrace her steps based on phone and credit card records and talk to witnesses to see what if anything they saw. Possibly after that phase is complete, they will move to some searches.