I have a 35' class a on the workhorse chassis and the 8.1. The best I can tell is that its rated to tow about 5-6k. That being said, my jeep is about 5200# on a steel tilt bed trailer so I'm probably around 7-7500# total. It tows it great and I can run at whatever speed I want and it towed the I-70 pass through Colorado just fine. I also have the allison 5sp with hill descent control.
Kitson has an older model (05 or so there was an upgrade) ford v10 and he also had no issues with the mountains. Although he did just rebuild his tranny.
The Endura super c's are nice and they have a Duramax with the 5sp allison but I think their tow rating is still in the 7k range. You won't get a 10k rating until you get into the diesel pushers and even then it has to be the larger ones.