They use a full-size queen mattress. When I bought my 2004 from the original owner, they had replaced the factory mattress with their Tempurpedic, but to sell it put some cheapie in there. It was horrible, so I swapped it with a Spring Aire and just replaced that one with a Stearns & Foster.
So if you are asking because you think you will want to buy a new mattress, any standard queen should work. One thing to consider is the thickness and how high it will be. In my Diplomat, the foundation is 22" off the floor. Add a 12" mattress and that is 34" off the floor. On our last mattress, climbing in and out at that height was hard on the edge of the mattress so with the new one we are using little step stools to make it easier on us and the mattress.