Recreational vehicles often have odd sizes or shapes of mattresses to fit into very compact space. When the original mattress wears out or gets damaged, it can be replaced. Here's how to find a replacement mattress for your RV.
If the mattress is not totally worn out or damaged, it might be possible to keep the mattress and just add a mattress topper to it. These come in a variety of materials with the memory foam being the most popular. These are not inexpensive, but they are available at discount stores and department stores in the bedding section. The foam type can be trimmed to any shape needed.
If the mattress must be replaced, check first at camping supply stores like Camping World to see if you can replace it locally. This saves on shipping costs.
Next contact the RV manufacturer to see if they can replace it and what the cost would be. For instance, if you have an Airstream trailer, contact the Airstream Company.
There are mattress manufacturers who make custom matresses for RVs. They can make a regular mattress of the size you need. Often the bed in an RV is a "short queen." They can make ones also with three square corners and one cut corner (angled). You have to send them the measurements and choose a thickness.
To find these companies, search the internet using the keywords, "replace RV mattress."