A 3 mpg you should be seeing clouds of smoke behind you, from unburnt fuel passing through the engine and into the exhaust. If not, it must be leaking out somewhere (which seems a more likely explanation). I'd start checking fuel lines and filters for leaks.
That coach has a larger engine and better tranny, but it also weighs a whole bunch more, roughly 10k more. Performance is pretty much determined by the weight/Hp ratio and both coaches are probably around 100 lbs/hp.
Per the brochure, the biggest 99 Bounder Diesel maxed out at 24,850. The CC Inspire specs out at more like 33,200.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL