While it is possible that you have only 1 tank and/or 1 dump valve, that would be very unusual if your RV has a toilet. Since the floor plan that I found shows a toilet and the secs sheet shows 2 separate tanks, I am very sure that you do have separate valves. Since the RV is now 5 years old it is possible that some previous owner modified it but such would be very uncommon. It is possible that you have a single valve the the opening which was added later with the two original valves between that extra valve and the 2 tanks. What you should have would look something like this picture. The smaller valve to the right is the gray water dump.Are they the same?