2007 Chevrolet Uplander fuel system problems

Have a 2007 chevrolet uplander van. When parked slightly downhill with gauge showing 1/3 full of fuel, vehicle will not start and will shut down if running. GM states that this is a normal feature of this vehicle and nothing is wrong. I've been stranded 5 times with this problem. What if the engine quits while going down a long hill, engine stops so no steering or braking and I run off highway, investigators would never look to see if the vehicle was at fault only that the old man ran off of the road.

Purchased used in november 2007 and shortly had trouble as when vehicle parked slightly downhill with fuel gauge showing 1/3+ vehicle would not start and gauges show zero fuel. Driving down hill engine will quit for lack of fuel. GM told me in writing that they know of this problem and considered it to be normal. Only yesterday was stranded in parking lot of fast food lunch place with gauge showing 1/3+ fuel level but car would not start. Fuel pickup is in rear of long 25 gallon tank.