We have a Coachmen 28QB on the Chevy 4500 chassis with the 6.0. We tow a Wrangler four down or a VW on a Demco Ss. The Jeep is around 4400# and the VW/dolly weigh in around 3900#.
The unit handles both well. More power would be good but, the 6.0 gets the job done. Long grades are the slowest. Out West, it isn’t unusual to get down to 55 on some of the longer grades. The short steeper grades usually mean greasing down and letting the engine run up to around 3600rpm’s.
At 3500#’s, I suspect your experience will be considerably different.
Hope this helps.
Gary
