I found one 2500 in Idaho. It had 373 gears and an 8.1 engine.
Sorry, I should have specified. 4.10's are the better gears for towing regardless of 6.0 or 8.1 motor, but the 8.1 with 3.73's is a great combo for what you say you'd need. If the 8.1 is in great shape and a good price, I'd buy it with 3.73's. Those things are tow beasts.
As far as trying to throw money at a 1500 to tow a 6000-7000+ lbs travel trailer up and down mountains and valleys, you'd be better off selling it and buying a 2500. 2500's came with the more stout 4L80E trans as well.