We replaced the mattress in our C with one from The Original Mattress Factory. Highly rated by Consumer Reports. They have stores in the Midwest but not sure how far beyond that or where you are located. All the mattresses in our house are from them. We got the exact same one as on our bed at home but "RV sized" custom made by them (took a couple days). Highly recommended.
'13 Forest River Sunseeker 3100
'04 Fleetwood Sedona / Reese Mini 350 / Prodigy / BAL
'94 Coleman Cedar