Loosing speed going uphill and revs at 3500 rpm+

OK, that`s what I thought, just wanted to be sure.

Yes, the class C shouldn't have an issue. It`s possible you have a plugged fuel filter. hard to say on the internet. I also know the trans likes to short shift, meaning it doesn't let you get the full benefit of its powerband. I used to have to shift mine manually to get it to tow good in the mountains. I then put a 5 star tune in it and it made it tow like it should.