[fourteener]

I grew up on the farm, and my Uncle would give me a long talk about lugging (low RPMs) an engine under tow. He said it was the worst thing you could do. It can simply 'pull the guts' out of an engine. Keeping RPMs low is good for fuel economy, but bad for pulling. You want that engine to 'spin like a top.' This is especially true for diesels.
Also, you never want to let an automatic shift from drive to overdrive constantly, as this has the potential to over heat your transmission. It is easier to simply lock the transmission in drive (or the next lower gear). On the first vehicles that had the tow button, that is all it did, it just locked the overdrive out (blocking that as an option).
That is my two cents, and I'm definitely no master mechanic, but I can tell you that I drive the Rockies all the time, and I'm not afraid to keep some decent RPMs when topping the Continental Divide.