They do make a product called flex Crete but it is highly specialized and only sold to the specialized and trained contractors so it is very expensive.
Why not look into a grayish tile and epoxy grout to get the look?
I don't recommend either but that is a better option than concrete.
What ever your choice of hardened material to use I can recommend stopping a 1/8"-1/4" short of any wall,framing or trim and use a silicone type of caulking or
Sealent to allow it to flex and move as the bus twist and moves while going down the road or trying to level it up in a camp spot but that last bit could almost go with any finish board wall idea because if cut perfect and the fit is perfectly tight then that board in the long run is probably going to pop through the screws and come loose?
Just my thoughts.
I have been in construction for quite awhile and I go into older buildings where the building has settled over the years and there are obvious signs of expansion and contraction(flexing) concrete cracking,brick and block cracking, Sheetrock joints and screws have released and falling off.
Long story short in my opinion especially for a skoolie it is safer to plan to the extreme for expansion and flex by leaving your boards,countertops or whatever a little short of perfect and hide it other ways like 1/4 round trim nailed through the wall instead of the flooring or color match the caulking to the area.
I cheated and made my own trim out of twisted and warped lumber that fit where I wanted it to be and I caulked the trim in instead of nailing it? If it pops off it means I didn't allow enough room for flex but it's an easy fix.
These are just my thoughts and ideas?
Wish you luck with yours.
Weight: A 1 ½ inch thick countertop made of standard concrete has an approximate weight of 18.75 pounds per square foot. (Granite is approximately 18 pounds per square foot.) Standard cabinetry will typically support this weight because it's distributed over a large area.