Many RV mattresses will fit standard mattress sizes. A very common RV mattress size is what is called a short queen. They are often 5 inches shorter than a regular queen. Some RV's and campers will have custom-size mattresses, when replacing this type of mattress there are a number of brands or factories around the country that can make completely custom-size mattresses. There is typically an upcharge for such a mattress to be made. Testing a mattress in advance of making a purchase is always adviced. However, this is more challenging on custom orders.