I have a 2005 Kountry Star with the 8.1 Vortec and 5 speed Allison. I tow a Chevy Malibu on a dolly, about 4,500 lbs. I have not had any issues on hills, but don't expect to fly up a hill in overdrive at 65 mph. Just returned from a trip to Hilton Head and went up the Fancy Gap pass on route 77. Ran the engine at 3,400 RPM in 3rd gear at about 40 mph. I had more pedal but didn't want to run the motor faster and there was no need as many trucks in the right lane and an older diesel pusher behind me were all running the same speed.
2005 Newmar Kountry Star 3742 - Gas
ACME Eze Tow Dolly
2008 Chevy Malibu Toad