Tour buses are made in all sizes, most as custom builds, but it is a different market from RVs. Those built in class C sizes are usually used by coach services that transport a lot of smaller groups for short tours, mostly with seating and small kitchen and or bar facilities.
Dynamax used to make some standard model touring coaches in the 22-28 foot range, which were sold through RV dealers but seldom purchased for dealer inventory. I'm not sure what they have in their current model lines, a new owner seems to be taking the brand more into conventional RVs.
What can be custom built depends on what you might be willing to pay.