I've never used the donut after our first trip, but I do have one. Our first travel trailer came with it. The dealership we bought it from owned a campground and we got 2 free nights of camping to try everything out. They did the PDI at the campsite. The dealer told me that it fits into the sewer inlet and then you put the stinky slinky into the donut. He did that part, so I didn't experience it. I bought something similar to
this after the trip.
One time I wasn't able to get the elbow to screw into the sewer inlet, the inlet was just too short and I couldn't get the threads on. I tried the donut, but it wouldn't fit, the inlet was too narrow. Fortunately we've never had that problem since.
I believe you can buy the elbow with the graduated threads separately. It makes it a lot easier than using the donut.
-Michael