Definitely measure it and then see if you could go longer if needed. A few inches over the plywood won't matter. I bought a mattress off of Amazon for my Cougar. It's not a true king. 2" narrower. On Amazon, search for Travel Happy RV mattress. Custom size. I paid $650 and it was at my house in two days. I'm pretty sure they have some of the common RV sizes already made up. They have 12" thick, 10" thick, and 8" thick. 12" is the softest. 8" is more firm. We went with the 12" and absolutely love it. My wife says she wants one for our house. Once you order the mattress, you shoot them a message with the size you need.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1