an RV is massively better insulated then a Truck cab. Still very poorly in general, but better.These sleeper cab A/Cs are likely too small for an RV, unless it is one of those tiny canned hams, and that might still be a problem. These units would likely struggle to cool down a hot truck cab, but their use case is to run once the truck parks so they don't have to idle the engine to keep the cab cool. In most cases the cab is already cooled down after driving for several hours.
My RV has 4 inches of foam in the roof. That's probably in the neighborhood of R16. While the Walls are built from foam - with around R7.
A truck is lots of metal with a R Value of close to Zero. The fiberglass section of the sleeper is probably around R2-4.
That's why a 25Ft RV is plenty of cool with 13.500 BTU A/C unit. You can buy those truck A/Cs up to 15.000 BTU. They sell those to Australia, those guys have 50 Celsius outside. That's 122 Fahrenheit.
I only have to cool from 90 to 78 fahrenheit to be comfortable and not from 120 to 78 - makes a huge difference.