As for the "Greatest American Novel", I like your call of Slaughterhouse-Five and would add to it A Farewell to Arms. Though, I might point out that while both of these picks have American Protagonists, they take place largely outside of the US (and partially on the planet of Tralfamadore) which makes it difficult to call them an "American Novel" in the same way one might coin Huck-Finn as such.
I liked Huck Finn a lot, I think I'll put that on my re-read list also (although I very rarely re-read books) - I last read that when I was a teenager, that would have been in the late 70's. Just thought of another book that I would consider a 'greatest American novel' candidate - "A Prayer for Owen Meany" by John Irving - such a powerful book.
aside: I used to always confuse John Irving with John Updike until I actually read John Updike. In my opinion, they couldn't be much different in their styles and subject matter. I started reading Updikes "rabbit" series and never got passed Rabbit is Rich.