The sofa bed in our RV had a good-quality inflatable mattress hooked into it. It had an attached inflator and all you had to do was unfold the bed mechanism, flip on the inflator and in a minute or so, there was a really comfortable bed. You couldn't feel a central bar, at all.
Of course, that's only as good as long as you don't spring a leak in the mattress in the middle of the night. You know what they say about inflatable mattresses......"When you actually want to sleep on the floor, but you want to do it slowly...."