I had a 18" exhaust extension fabricated by a local welder. I bought an exhaust pipe from Amazon then had a new flange welded to it. The blower pushes the exhaust and heat away from the rv, so the exhaust extension only needs to 'steer' the exhaust flow so the air flow from the blower takes care of getting the fumes away. Here's a link to the extension I bought on Amazon....
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1