We have a 2012 Discovery 36J with the Cummins 380 hp engine. Just came back over Wolf Creek Pass, towing our 5000 pound Acadia SUV. No problems at all, and that's at about 10,000+ feet elevation. A great engine/transmission (Allison 3000MH) combination.
I'd be surprised if you would face anything more difficult than that. We've only had our coach since January, but it's been wonderful. Every manufacturer will have issues, but there's been nothing that our dealer and Fleetwood hasn't taken on to correct.
Good luck with your search!
Joe and Debbie, Emma the Aussie Cattle Dog who adopted us
2012 Discovery 36J, Blue Ox and Air Force One, 2010 GMC Acadia Toad
USAF (Ret) WDØM https://wd0m.com