[Richard]

I recently had to replace the muffler on my 99 Newell. The original was the Riker model as documented in other threads. I could not find the Riker on line, and I wondered if there was a financially efficient solution than ordering one from the mothership. 
   
   

I did not want to use a straight pipe, and I really didn't want to do any major welding or modifications that would prevent the use of the original style Riker. 
   
I opted to use this round muffler. https://www.amazon.com/gp/product/B00F2N...UTF8&psc=1
   
You also need a elbow to transition out of the existing flex tubing into a horizontal run. https://www.amazon.com/gp/product/B01N65...UTF8&psc=1

You need two clamps  https://www.amazon.com/gp/product/B073ZD...UTF8&psc=1
     
   
And a short piece of round tubing to make an exhaust tip. https://www.amazon.com/gp/product/B079VF...UTF8&psc=1

I used an angle grinder to get the length and angle right.

I used two generic muffler hangars to support the muffler at the elbow, and at the exhaust tip. Total modification to the coach was to drill one hole for the aft hangar. 

Initial results are the sound is no louder than the original muffler. I will report on maximum turbo boost, fuel mileage, and jake brake sound when I have some miles on this setup.