Jake brake on or off driving on flats or small hills?

"Most motorhomes..."??
I have owned three Class As and a Class C.
One of my sons had a Class C.
None had any kind of "grade retard" or retarder.
In some municipalities an UN-MUFFLED compression brake might get the driver a ticket. A MUFFLED compression brake usually will not.
An exhaust brake makes very little noise. I have never heard of anybody getting a ticket for using one.
Not all compression brakes are Jake Brakes, but ALL braking systems made by Jacobs are Jake Brakes according to the Jacobs website.