hlagooty, from my Prevost owning experience and reading, those engines get about 5.0-5.5 mpg.
Not to steer people away from this excellent RV site but its not really Prevost based. You need to join 'www.prevostcommunity.com' to find ALL of what you need to know. The site is free but you have to join as they don't want spammers.
If the coach your talking about has been maintained, that will be a major plus. But if it has been sitting, plan on big bucks to bring it up to snuff!
You didn't mention if you have the ability to do any of the chassis work yourself that WILL be needed, if not, I would probably shy away from an older coach because parts and knowledgable mechanics get scarce as they age.
There are company's that can and will rebuild cruise-air's. The problem with cruise-air's is that they don't work wonderfully when the ground is hot. People report that they will run water on top of the pavement where the cruise-air pick-up air return is located so that cooler air is pulled in. I had 4 roof-top units that work much better and are less finicky.
Good luck and let us know what you decide.
2021.5 Pleasure Way Plateau FL Class-B Sprinter
2017 Lincoln MKX, AWD, 2.7 V6 335 hp twin turbo
(Sold) 2015 Prevost Liberty Coach, 45ft. 500hp Volvo