Yeah I will also say that the Canon 10-22 is a good rectilinear UWA lens, and I really like mine. If the fisheye effect is what you want to avoid, then this is definitely an option you should consider.

If you want to save weight, shooting multiple exposures at 18mm with your 18-135 and then stitching together in Lightroom afterwards will give you a large field of view and a nice high resolution image. But the 10-22 is a pretty lightweight lens as well!