Maybe just a little but when dealing with large sums of money it is good to be cautious. If his CU in their letter says they will loan him less money than you want for your RV, then he can't afford your RV. The letter has nothing to do with your RV, it is the amount that the CU feels comfortable loaning him for any RV. Generally, pre-approval letters are written without a particular RV in mind. He tells his CU how much he wants to borrow and they check his credit and either agree or tell him how much they will loan to him. After he finds a particular RV, they will do their diligence as to what the RV is worth and if it fits their lending criteria. All a pre-approval letter is for is to show a seller that he is qualified to buy your RV. Evidently he is NOT qualified to buy your RV so it is time to move on from this buyer.
My suggestion to meet the buyer at his bank or CU if you hold title free and clear was a protection for you from counterfeit bills or phony cashier's checks. If a buyer's financial institution is unable to go be gone to, then you should make other arrangement to complete the sale. Suggestions would be cash (meet at your bank and deposit upon receipt) or a money wire transfer.