Hi Traci,
I'd seriously consider the DA 55-300. This lens would be less than 1/2 the weight of the DA 60-250 (470g >1130g), and about 2/3 the length (11.2 cm > 16.8 cm) for backpacking -- and about 1/3 the $$$. With the K-5's high ISO performance, I've found that lens speed is a whole lot less of a consideration in my shooting, though premium fast lenses are almost universally optically superior. Stop down the DA 55-300 (and absorb the difference with ISO), and it's pretty sharp -- not as sharp as my FA* 300/4.5, but if I'm going to carry the camera all day, it's a sacrifice that I'm willing to make in many instances. It will also not overlap your 18-135 as drastically, but as you suggested, this really isn't that much of a consideration, since I usually can guess pretty well which range I'd be most likely to be shooting given a particular situation.
A few recent examples with the DA 55-300 at 300mm (admittedly at close range). Both were shot with the K-5, DA 55-300, f8, 1/800, ISO 5000. Both were shot in jpg ***, light NR in PP with Topaz Denoise 5, light deconvolution sharpening only with Topaz InFocus, and resized for the web.
Another advantage is that the screw drive DA 55-300 can be stretched with a 1.4x AF TC in just about any decent light outdoors (note that the Sigma EX 1.4x APO or APO DG TCs will not mount on the DA 55-300 because of the protruding front element on the TC). With SDM default, the DA* 60-250's AF might be touchy with PZ compatible TCs (there are no TCs that can claim true SDM compatibility).
All that being said, I can't fault any Pentax * tele lens or zoom, and own a few. They certainly get the call if I'm going out specifically to shoot birds with the CF tripod and Wimberley gimbal.
If you're anticipating shooting very long a lot, the DA* 300/4 or the F*/FA* 300/4.5 with the Pentax F 1.7x Auto Focusing adapter is a formidable handholdable 510mm combination, and the extra speed of the lens itself allows the extra magnification. I've seen examples from the DA* 60-250 with the 1.7x AFA, and I would definitely consider this combo if the FL range (102-425mm at f6.3) was more useful to me -- I'm generally pretty much a "longer is better" kind of shooter.
Here's an old one shot with the K100D, FA* 300/4.5 +1/7x AFA, f8 (wide open at the lens), 1/400, ISO 200. Shot in jpeg highest quality, lightly sharpened with Focus Magic, and resized for the web.
Scott