Any medium sized furniture store that sells bedding has (or should have) access to a bedding manufacturer that can make custom bedding of any size. This is if a standard size mattress won't fit..... or won't fit right. The cost is not much greater than a regular mattress. By getting a custom fit you have your choice of firmness, ticking, etc. You need to measure closely because you don't get a second chance at your dimensions. A lot of the RVs today do not have standard sized beds in them.
Our coach has a back north/south king bed and we chose a latex mattress. The mattress folded just enough for us to get it down the hall and it is very very comfortable.
We have the matching bedspread that came with the coach but it's tucked away in our closet at home. The reason being we have TV in the coach bedroom and we are all the time laying across the bed watching TV. We have a nice blanket on the bed and it looks nice when we make it up every morning and we don't mind messing it up during the day.
Just my comment.
2008 Monaco Dynasty, 42ft, 4 slides, 425hp clean air Cummins diesel
2013 Jeep Wrangler Sahara Unlimited w/M&G Braking System
Roadmaster Blackhawk 2 All Terrain Towing System