RV Repair: Gray Water Tank, rubber collar, water tank

Question
I developed a leak in the ABS piping leading into my gray water tank. As I was repairing the leak, I noticed that the pipe leading into the tank was moving. I discovered that it was not glued, but has what appears to be a rubber collar in the tank that allows movement. Is this common? I want to repair this correctly, and do not want to glue it in unless I need to. I can not see the connection, it's all been by feel.Thanks  

Answer
I need a bit more info. How old/new is your unit? I have never heard of this rubber fitting that you speak of. It may be a new technique, or an old one. Does it look at all like it was glued at one time?
All the connections I have ever seen are always rigid ABS black pipe glued together with ABS cement.