A National is nation wide. If it is for the state, it is a State Championship.
I believe that 4-H offers state championships. I don't know about Nationals. That information might be on the 4-H web pages.
There are no APA approved state championship shows in my area for poultry, so if such a thing exists, it isn't in every state. There is a Nationals for poultry. Once a year and it is a huge show with people flying in from all over the country.
I've not heard of any shows just for ducks. A breed club might give their national at an open show, so the duck specific breed nationals would just be a sub-set at a bigger show
If you get a subscription to The Poultry Press, all the approved shows are listed.