We also suffered with our original RV King with a plush topper to make it a little better. Finally, a few years ago, we were in Fresno for a while and went to a custom mattress maker in town, and after trying out a number of different mattresses in their showroom, had a latex mattress made, stiffer latex foam in the middle and a bit softer latex foam on both top and bottom, (makes this mattress able to be flipped occasionally), and it's also an RV King with the radius corners. We have a high quality mattress pad/cover that makes it real cushy and we sleep like babies. They can make anything you desire, as firm or soft and out of anything you want, and much higher quality than ordinary mattresses. We've been very happy with our purchase.
https://abetterbed.com/