Bought a used coach that came without the sofa. We have the two of us and our son using the motorhome just about every weekend of the summer. We have 72" of width available in the slide for the sofa...
If you have a large window that can be removed...I've seen sofas go in through the window opening. Some sofas have removable arms or other parts to make it easier to get through a doorway.
If you're handy, how bout building a sleeping platform with stiff pillows to lean against when sitting.
Or use a futon and raise it up to make it more comfortable for sitting.