I would look at IKEA. They have some cheap and not-so-cheap sofa options, some sofa sleepers, some unfold, but the good thing is that most of them can be shipped in flat boxes and put together INSIDE the MH. Most of them are firmer than a regular sofa, but they are comfortable.
I am thinking about taking our sofa bed out of the 5'er and replacing with a couple of comfy chairs. We have a bunk house that will sleep 9 without the sofa bed, so it is pretty useless. It hasn't been pulled out since we purchased it two years ago.