It depends on how far you are going, how much you are hauling, and what the terrain is like.

I've toured with an unmodified road bike (53/39 chainrings and 11-23 cassette, 32 spoke wheels) towing a BOB in Florida with no problems, for example. Possible modifications for heavier duty might be: beefier rear wheel, lower gearing, kevlar tires or tire liners. With a triple chainring and stout 36 spoke wheel in the back, you could tour anywhere.