CR, I hate to say it, but 6mpg is about as good as you can hope for... probably. The new bus likely has three tons over your old one, one extra axle (two more tires of resistance), a bigger front profile, and almost 200ci's more cylinder displacement. Not to mention the slushbox.
But, you're right... there's more to life than MPG. If you like the bus, and it suits you, just fill the tank... and try to smile.
Here's some math: if you drive 25k miles a year touring (do you???), and fuel is ~$2.50/gal, the diff. between an 8mpg coach and your 5.8mpg one is about $3k. ~$3/gal makes it more like $3500. Is your new bus worth that to you? Might be... and only you can decide.
If it was all about MPG, we'd all sell our (fill in the blank)
and buy an old RTO Crown. 10-14mpg, I'm told. Amazing...
Alternatively, the "toterhome" RVs are built over OTR truck chassis and can handily get 10mpg... and pull a 40,000# trailer, if desired. All shapes, sizes, and price ranges. Some even have slides! Big Dawgs, for sure. Could haul a few Hawgs, eh?
Meanwhile, Hang tough! Burn that petro while it's still relatively cheap!