You can't siphon a trucks gas just because it has a blue line flag. Most modern cars have an anti-siphon valve, so that causes issues.
Instead, you locate the fuel tank and drill a small hole in it. That way you can let the fuel collect in a tub or jug. On lifted trucks this is even easier. Drill the hole, pop a large jug under it, perhaps use a small rubber hose or funnel.
Remember to leave them a little note that you've drilled a hole, so they can go get it repaired and not leak fuel everywhere attempting to refill it.