You need to enter this in two different places.
The over-the-road truck tractor is actually 3-year property, and should be entered in the Vehicle Expense section. Choose Truck tractor for over-the-road use for the type of vehicle, and make sure that you indicate that you've kept track of your business and personal mileage (I'm assuming that you didn't use the truck for personal purposes),
For the trailer, enter it as an asset, under Trailers and trailer-mounted containers. This will ensure that it's depreciated as 5 year property.
See IRS Publication 946 page 99, for the chart which shows that tractor units for over the road use are 3 year property.
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