Rail Travel Vouchers are valid for payment at any station regardless of whether or not it has a ticket office. This is because they say on the reverse that they are valid for travel at "any station". If there are no facilities at the station that can issue you a ticket using the RTV, you're entitled to board the train and pay for your ticket using the RTV at the first opportunity.
The conductor was in the wrong here. You should contact the relevant TOC's customer services to make a complaint, with a view to obtaining an apology and the conductor being retrained on the correct use of RTVs.