The problem with coil spring mattresses is the rigid frame makes shipping a bit difficult and sometimes getting the mattress into or out of the RV can be difficult. If it won't fit through a door or turn a corner to get to the door then a big window would need to be removed.
This is why most RV mattresses are foam or air mattresses.
We live near San Antonio and like a great many cities we have a custom mattress manufacturer here in town. This really used to be the way people bought mattresses in the "old days." This makes it easy to custom order a mattress of any shape and size for RVs and boats. For a previous RV we ordered a custom Short Queen locally from our mattress manufacturer and got to pick the materials, the number of coils, and topper. They delivered the new mattress to our RV and took away the old one.
You're in Southern California and I'd think that there would be dozens of such manufacturers around the Southland. But when I search via Google I only found one.
https://customcomfortmattress.com/. Their nearest storefront is in Brea.
There are many online custom RV mattress companies and the largest is probably Mattress Insider:
https://www.mattressinsider.com/ - but I think they only sell foam mattresses.