Yup if you don't have the elbow connection for your sewer hose the CG will gladly let you buy the sewer donut from their little store.
That said some CG's do not have a sewer connection that you can tightly screw the elbow connection into. Then you DO have to have the donut.
I carry one with me even though I have the threaded elbow connection for sewer connections that are not the right size or not threaded.
There is nothing as bad as being parked next to someone that just opens the sewer pipe and throws the hose in letting sewer smells emit all over the place and smells even worse when they go to dump.
I appreciate CG's that make it mandatory to have the elbow connection or make you buy one of their donuts! :C