I had posted about my new setup that allows for an airtight connection. Even though we are only draining grey water I got into trouble at one RV park in CA for not having an airtight connection into the sewer.
I now use a standard 3 inch slinky hose and a 90 degree threaded elbow for the sewer pipe connection. Why 15 feet? Well, seemed like a good compromise between 10 feet and 20 feet
BTW, this setup works much better than a regular garden hose because it doesn't get air locks, which can be a nuisance.