I'm a Bus. Grad. maybe I can offer a dimes worth of insight into your Loan Officer's insight. Banking Regulators expect the loaning institution to maintain a 20% margin between the value of what you are buying to it's ture market worth. In theory, if you default the next day after the loan, the 20% buffer will pay for the banks marketing and selling costs to recoupe it's money and make it whole.
There are no established market values for older items like your trailer ...as there is not a standard for a 1972 Dodge Challenger....as they both depend on model and condition.
If you get two market knowledgeable people in the used RV business to write appraisals (about $25 to $50 dollars each depending on the market) and your loan request is 80% of the apprasied value of the unit, your load officer will have documentition upon which to make the loan. The rest will depend upon your credit rating and your past payment track record with the bank.
Value is in the eye of the beholder. Banking regulators require the bank to document it's effort to make sure the loan will be repayed in full.