Additional axles on customer-owned chassis within the Six Month Rule

We are a truck body manufacturer that sells bodies at retail. There are times we install a taxable body on a customer-owned chassis on which we also install one or more additional axles. The customer-owned chassis have a GVW greater than 33,000 pounds before we add any axles and the chassis are not more than six months old. We pay excise tax on the bodies we sell at retail, but we do not pay excise tax on the axles. In connection with an audit, an Internal Revenue Service agent is claiming that we are liable for excise tax on the axles we sell because they are sold in connection with our sale of taxable bodies. Is the IRS agent correct?