You mentioned "one" rental. Is that one rental the only part of your business? If not, can you clarify what you were referring to by "one" rental? And if you have multiple rentals, you can't pick-and-choose where to add the deduction - it must be applies proportionately between all rentals that used the truck.
Section 179 only applies to a "Trade or Business". In most cases, only one rental is usually not considered a business.
If your activity does rise to the level of a business AND the overall business activity on the entire tax return has a profit (which would include all rentals, W-2s, and self employment income), then you need to make an adjustment to Line 11 of Form 4562 because TurboTax automatically assumes that rentals are NOT a business.
