Sugino makes some popular road touring cranksets, but they don't come standard with as low as teeth as the standard MTB crankset you are looking for, but they do come with a lower chainring setup off the shelf than Tiagra or 105 does. They have 110/74 bolt center diameter and come off the shelf with 26-36-46 chainrings. One of these Sugino cranksets combined with a MTB derailleur and MTB cassette on the rear makes for a nice touring setup on a road bike. You could also potentially keep a road 12-27 cassette if that is what you already have, and replace the front 26 chainring with a 24 chainring to get a lower low gear, although some folks on here might argue that that is still not a low enough gear. There is also a gear calculator at Sheldon Brown.

http://sheldonbrown.com/harris/cranks/11074.html