Toyota Sienna was traded in on 3/31/18 when Ram truck was purchased. Both vehicles were solely used for business. Do I need to enter "Basis for gain/loss"?

TurboTax will calculate the gain/loss for you; you can leave this entry blank.

When you Edit your Vehicle in the Business Expenses section and indicate that you 'stopped using it in 2018', enter the Trade In Amount you received as Sales Price.

If you have been claiming Actual Expenses for your Vehicle, the basis for gain/loss will be the remaining basis after prior depreciation has been subtracted.

If you have been claiming Standard Mileage expenses, you will need to calculate and enter the 'Depreciation Equivalent', and TurboTax will calculate gain/loss (screenshots).