I use a 4" aluminum roof gutter downspout with an elbow epoxied on it. The elbow goes over the generator exhaust pipe. After you glue the elbow to the 10' length of downspout and it hardens, measure from the end of the elbow to the other end of the downspout, divide by two, and cut the assembly in half with a Dremal tool. Put a couple of rivets on the half with the elbow about 2" from the straight end. Slide the straight piece over the elbowed piece down to the two rivets, and you have a 10' smokestack. Hang a 10" loop if string from your motorhome gutter to stick your smokestack up through to hold the smokestack upright. Wrap a rag around the exhaust pipe so the elbow doesn't touch it; it makes much noise if the metal downspout is touching the metal generator exhaust pipe. Don't use very much rag - you want the air to flow into the elbow in order to generate a venturi effect.
This really works great! I've been using it for about two years and we never get any exhaust fumes inside to coach any more. I do close the roof vents while using the generator. My exhaust pipe sticks up about 2' above the roofline.
