If shifting to higher rpm and/or slowing down on a grade is a problem when towing, then forced induction, diesel and big blocks are the only options. All n/a gas motors are going to shift (or slow) on grades when pushed. The DOHC motors that Toyota uses are more prone to this than, say, a GM push rod, since the torque curve is pretty steep up to ~3k+ rpm, but that's how they're intended to work. They rev so quietly, that I don't see where anyone would mind? Not much louder than a turbo spooling, to me, and pretty much always quieter than a Cummins :)