I have a Tiffin on a FRED.  What everyone is saying for the most part is true.  Louder than a rear engine.  But, not too loud.  I just pulled everything apart and reinsulated and sound proofed the engine bay.  Made a huge difference.
Leaf springs, not air suspension.  So, the ride is rougher.  But, Koni shocks help.
Mileage is great.  I can get 9 mpg towing a trailer.
It is a compromise.  Slightly better than a Ford V-10.  Not as nice as a DP.  Perfect for what we were looking for.
My next rig will be a DP.
Good luck.  Ask me any questions you want.