As a frequent trekker/backpacker in southern Europe, I never go out without my 7D, 17-55 and 10-22.
Though I never used the 15-85, the rave comments on this forum suggest it's on par with the 17-55, with added range.
5.6 should not be an issue : you'll stop down outdoors, and rely on IS in churches.
For a single lens setup, I'd be inclined to second the choice for the 15-85.
...Unless you believe you'll get your nose on the southern European walls.
You'll just be glad to fit a whole basilica within frame using a 10-22.
I would not recommend too much the 10-22 for nature : 10mm is so much wider than my brain is aware of.
I have been lucky to have a perfect match between my 7D and 10-22. Apart from stretched sides at 10mm mine is as sharp and colorful as I could wish, straight out of the box, on 7D and 450D.
If you can afford the purchase, a 10-22 is a great second lens to carry, being light and compact.
I find I rarely ever go longer than 100mm, even though I do lug the awesome 70-200 2.8 around.
Faced with the pictures I brought back, I never regretted carrying my cumbersome 7D.
A 10-22 ends where a P&S starts, so you get unique/different pictures.
Best of luck with your trip !