No, you cannot deduct the principal you pay for the truck. The truck is an asset of your company.
You are correct to deduct the interest you pay on the loan on the truck.
You should depreciate the truck as a depreciable asset of the business. The ordinary depreciable life of a truck in a business is 5 years.
The Tax Cuts and Jobs Act of 2017 allows Special Depreciation of 100% of the asset value in the first year if you so choose.
TurboTax will guide you to choose how you wish to depreciate your truck this year.
Deducting Depreciation on My Truck and Other Assets
