I'd have to look at the EVAP system but on the systems I get to screw with, slow fill = vent solenoid or it's filter is clogged. USUALLY its a cobweb though
There may be a tube running alongside the fillerneck. Follow it back to the first connection you can get to and blow air though the part that goes up the filler neck. I'll look tomorrow and see how the system is on these trucks, it may be different.