using a futon mattress instead of cushions

[tinsltootsie]

We just purchased a 74 overlander and are working on many things, though it is in very good shape. At some point I want to redo the cushions and think that a futon double mattress might be the best option. I think it will fit, but don't know how well it will stay folded as a couch. We have a futon, but it has a slant which sits the cushion in, the airstream couch does not. Any one who has tried this? I have read where it does interfere with the curtains. this is not a problem. Thanks for any help.