Shower mixing valve

You did not look very far. Delta and Moen both have them. Lifetime warranty. Both were posted on your other post.

Just the other day someone ask about a problem with either hot or cold but no comfortable showers.

So I searched for and posted links to two different thermostatic shower valves

In fact "Thermostatic shower valve" was the search

One is "Surface" mount for hand held showers, the other was "Behind the wall" for fixed head showers... I thought I bookmarked the links but alas, No joy there,

You asked this question five days ago.
Your previous thread is
https://forums.motorhomemagazine.com/index.cfm/fuseaction/thread/tid/29205741.cfm

For RV, probably no one. Might be able to install a residential one.