Basically just will not start when parked on a hill. A mechanic thinks it is fuel related. It is my mother's, and the first thing I asked her was if she had fuel in the tank, and she said she did. Please help!!!!!!!!!!

To know for sure, you will need to find out if the engine has spark and fuel..this will tell you what system is causing the problem.
Most starting problems related to the angle a vehicle is parked can be related to the fuel system.
More-so if the tank is near empty.