We have a Independence, so I cannot comment directly on fit in the Spirit, nor do we change the configuration from day <-> night, as we chose it for the twin bed option. However, knowing I'm a twitchy sleeper, we went directly to the Southern Mattress option, and got the extra layer of soft foam on top of their most popular mattress. Initially, the Casita crew, not knowing that we had ordered that extra layer of softer foam, had put them in upside down. My husband was perfectly comfortable, but with my arthritis, I was not quite comfortable. After a couple of nights of restless sleep, it occurred to me that we might have them upside down, checked the lower sides to determine if they were softer, determined that they were, and we switched the mattresses. My husband is still comfortable, and so am I.
The other option we considered was to get an IKEA natural foam, latex mattress (like the one on our bed at home), and cut it in two with an electric knife. Many people find they don't need to cut foam mattresses to fit the wall, but those who do use the original pads to make or as a template. When it came down to it, though, neither of us wanted to be the one who (potentially) screwed that up, since the split would be facing into the aisle. The other issue was that of having a cover that fit properly, and firm edges, which the Southern Mattresses do.