I thought when you bought a truck period there was no sales tax- you pay FET. I've only bought a truck in Ohio, though. So maybe some states charge sales tax in addition to the FET on a truck purchase?
Now on parts for truck, some states have sales tax exemptions- like when you buy tires or if you have work done. We always try to buy in a state that gives truckers a sales tax exemption.
Sales Tax Exemption Article