Cummings ISB 5.9L Turbo Diesel 300hp

I'm looking at buying a 2002-2005 Winnebago/Itasca 32-footer with the 300hp (several possibilities at the moment). It will be towing a Chevy Tracker, with lots of mountain routes - the 32-foot maximum is to get into some odd places in the Yukon and NWT in particular. I've already ruled out the 275hp models as not having the power I'll need (ie the Winnebago 32T), but the 300hp will do the job, I think. I spent the last 35 years of my working life driving diesels, 350 Cummins and old Detroits, so have a decent knowledge of the pros and cons. The big question is the $/mileage one - what sort of mileage can I buy and still expect to put lots more on before the big bills for maintenance start arriving? My thought is that if I can get in under 120,000 km (80,000 miles) for under $55k, it will work out well. Any thoughts on that will be appreciated.