From the Keystone website FAQ's:
Why do I have to fill out a W-9 form for a reimbursement?
As a subsidiary of Thor Industries (a publicly traded company), Keystone RV Company is guided by a number of Thor policies. One of those is how our company documents outgoing payments. While our company's payment to you may or may not be a taxable event, it is our policy to have a Federal Identification Number for every payee, which we collect via the W-9 form. This helps us make sure our books stay in compliance. Note: By providing this information, it does not automatically indicate that any payment you receive from Keystone would be subject to Income Tax, however, Keystone cannot provide you with any tax advice so we would recommend you discuss your specific situation with your tax professional to determine your tax liability, if any, as a result any payments from Keystone RV Company.