Two suggestions: ONE I have done
If you are in/near DETROIT, MI. (The west side) then there is a company on Telegraph, East side of road, between 5 and 6 mile roads. Tri-County Matress.
On one of my trailers they took measurements and custom made a new matress for me. Reasonable price (though the fabric pattern was .. Well chosen by price by me so I can't complain) It was good and FIRM (just what I ordered) NOTE they can make 'em pillow soft or firm or .. Well, my choice is basically "Concrete Slab" and they did a good job.
My current matress is 16 years old and when I get the engine replaced, and the tires replaced, it's one of the two top items on the agenda, I will likely re-vist Tri-County.
NOTE: There are many such companies, I have dealt with this one.
Next: A Internet firm, Casper.com. Do not knwo if they do "RV sizes" but do know this, Google HAM NATION.. Watch an episode or two and you will see Don talk about Casper He will give you a discount code you can use.
That (And Don's E-dress) is all I know about them.
Oh, they ship you install, 30 days later decide if you re-box and return or not. That tells me they think they have a good product.. but I've not tried it.