You can see such setups at most campgrounds with permanent or seasonal sites. As best I can recall, most of the ones I have seen use schedule 40 pipe with the appropriate fittings. A piece of slinky goes from your dump valve to a Foam Rubber Sewer Seal - Item - Camping World.
to create a flexible joint. This is inserted into a 4"x3" adapter which is glued to the schedule 40 pipe. The dump end usually consists of a 90 degree elbow that fits into the campground sewer, perhaps using another foam rubber seal to make the whole thing airtight. Look around before trying this so you can verify my recollection of how it is done. Be sure you have enough pitch so it will drain properly.