Sofa Cushion -- I agree with Usagi's comment. I have a 2004 17 footer. I just replaced the seat cushion for the sofa, with higher density foam from an upholsterer. It was expensive, but for my back, I needed more support when sleeping. Joann's Fabrics sells foam, probably higher density than the original and in the 5" thickness needed for your missing cushion. If you can't sew a cover for it yourself (I certainly couldn't) I'd bet the people at Joann's could put you in touch with someone who could do it for you, an experienced person, and probably altogether much less than what I paid for foam alone.
The dimensions for the back-of-the-sofa cushion are mostly the same as for the cushions at the back of the bench seats at the table, except for length, which is exactly the same as the seat cushion.