We received a Southern Mattress as part of our purchase of our one-prior-owner 17 SD. Once we (I) got fed up with the twice daily table to bed to table conversion, and came to terms with using the side table and benches the Southern went it replacing the
Casita bench padding. The SM is much more comfortable for sleeping and the side table is more than enough for the two of us.
The SM isn't perfect, and I'll point out what I feel are deficiencies so that you can use that insight to evaluate other mattresses you might be considering.
The SM has curved corners that match the Casita's corner, BUT!.... the
Casita is still curved under the rear window, the SM is not. That leaves a few inches of gap directly under the window. Since I sleep against that wall I too often find myself waking because my hand has dropped in the gap and gotten wedged enough that I can't turn while still asleep.
I think the SM is too thick. A thinner mattress, maybe 2/3 or even 1/2 as thick, will be enough to be superior to the
Casita seat cushions / bed padding. The mattress would be lighter, easier to bring into and out of the trailer on each end of off-season storage, and would save some rear
weight bias when traveling, and would be easier to climb up into and down from.