1. Make sure the gas cap is good.
If it is then the best way to find a leak in the EVAP system is with a smoke tester. Take the truck to a shop with one of these machines. It's a pricey way of getting the leak found, but on the other hand it can save hours of aggrivation with vacuum pumps trying to accomplish the same thing.
Basically, low pressure smoke is pumped into the intake (vacuum) system. All possible points of exit are blocked off (e.g. air filter housing) so that there is a true air-tight environment. Any leaks in the intake, and thereby the fuel EVAP system, will be shown by the leaking smoke.
As long as the gas cap is good, they will be looking for leaks predominantly in the lines going to the charcoal canister and the filler neck tube.
Good luck! Sometimes finding these leaks can be a real pain ...