I'll speak for a FRED on a Tiffin. I've been very happy with my FRED. First people might tell you it's loud, and it is somewhat loud around town, but when you get it on the highway, it's very, very quiet. I think every FRED comes with a Cummins ISB engine. My best fuel mileage has been 12.8, worst 11.1 towing our Jeep Cherokee. Also, with the Tiffin on a FRED, you get a flat floor between the driver & pass. seats. A new (2010) model will have a UREA tank (the fluid you have to add every 3rd fill-up) and yes, there is a large price increase associated with that. Another good point is that the FRED has an exhaust brake. While my coach is an '08, the additional cost of the FRED over the Workhorse back then, was easy for me to justify. With the added cost of the '10 emmisions, I'm not so sure I'd be able to afford/justify the deisel/FRED option. If it works for you, I highly recommend a FRED. Oh yeah, I have a 75 gallon fuel tank.
Drive 'em both, crunch the numbers & good luck. If you live close, heck, I'd let you take my rig for a spin!
2012 Thor Astoria 36MQ
Butter Toffee on Toasted Almond
'96 Cherokee toad Tim, Dar & our black Lab "Chase"