Auto parts stores have various diameter & length exhaust pipe pieces, including elbows. I made an extension for mine as part of a home made "Genturi" type exhaust that channeled the exhaust above the roof line. It just slipped over the existing exhaust when needed.
You probably don't need a tight fit, some any leakage will be outdoors anyway. Also, the hot gases will tend to follow the path of least resistance and flow out & up the pipe. Just avoid any restrictions. Small genset engines do not handle back-pressure on the exhaust at all - the engine stumbles and dies.
If you want a permanent mod, you can have a new tube made - it's a bolt-on installation. However, a pipe that sticks out more than a very few inches is a real nuisance when walking around the coach.