From what I read about Trump's business, I believe most of Trump's loans are commercial real estate loans in which the business is lent a sum of money to invest in the business. The collateral is not a specific piece of property but rather the assets of the business.
Yearly the Trump Organization creates a Statement of Financial Condition in which all of his properties are listed showing current value of assets and offsetting liabilities. It is this document that is attached to his loan applications, applications for insurance, negotiations with business partners, and other financial disclosure requirements.
Since Trump owns fully or partially dozens of properties in various states and other countries, any bank that attempted to obtain a complete independent appraisal or all his business interest would see the loan go to a competing bank long before the appraisal was complete. Therefore the banks use his statement of financial condition to make an estimate of net worth. Knowing that Trump grossly and illegally exaggerates his assets, the bank does a haircut on his net worth to come up with an estimated market value of his assets.