Note that there are several different designs of the transit bars.
But that said, it is a bar that goes between the wall of the motor home (inside) and the outside side of the inside "Edge" of the motor home (About the best I can describe it)
Mine is a long metal tube, with the common push button and move it to a different hole type of length adjustment. It has threaded bolts in each end, one ends in a "T" handle the other is slotted, the slotted end fits a socket on teh wall and the "T" presses against the back side of the inside trip (Frame) to force the thing to remain IN all the way even if the brakes fail or the pins shear.. Which they used to do.. Often.