A few pointers/thoughts.
1) Focal lengths are always the same, they never ever change. A 50mm EFS lens will give you (on a crop sensor body) the exact same photo that an EF50mm lens will as a zoom will set to 50mm. The 100mm will give you the same shot as your 18-135mm set to 100mm on its zoom scale. The idea that it gives a longer focal length is theory only relevant to people who have used 35mm film/fullframe digital camera bodies and amounts to giving them a basis for understanding the angle of view that the same focal length will give on crop sensor as opposed to the 35mm. For those who have no experience of using film or fullframe cameras its a moot point and not worth worrying about.
2) Is your 18-135mm your only current lens? If so if you sell it and get the 100mm you'll be limited to just 100mm. You might find this less than ideal in many situations so I'd advocate keeping hold of the 18-135mm (100mm is really very long if you're shooting, say, indoors or want to do a wide angle vista shot of a landscape (though you can of course use photo stitching with a longer lens for this)).
3) If you want a longer zoom lens for subjects like wildlife in yellowstone you'll be wanting 300mm at the very very least - with longer being ideal. Sadly this does cost and getting quality and long reach is what makes wildlife (and sports) some of the more expensive areas of photography. Of course good field craft can let you overcome a limited reach as can compositions that don't rely upon the subject filling the whole of the frame.
4) You might want to consider a Sigma 150mm f2.8 macro original (second hand) or 150mm f2.8 OS macro (newer, with OS which is like canons IS and also a bit more expensive than the older one). These two lenses are very popular with nature photographers in general, offering a bit more reach, a bit more working distance over the canon 100mm offerings. In addition they will also fit to sigma brand teleconverters - a 1.4TC gives you a 210mm lens which hardly has any loss of sharpness - a 2*TC gives a bigger hit to optical quality and loses auto focus, but gives you a good quality 300mm lens (not anywhere near as fast with AF as a regular 300mm of course).
There is also the older sigma 180mm f3.5 macro (second hand) which is pretty much equal to the 150mm, save for being slightly longer in focal length and a little heavier (optically speaking there is really nothing to tell the two apart).
In the end what is best to go for is going to depend a lot on what you want to shoot, how you want to shoot and how much budget you can get together for the trip. If its a big one time trip you might also consider renting gear - LensRentals are a top choice for the USA and there are one or two other rental companies - this would let you rent better glass for the trip (though I would advise trying to give yourself a little leeway at the start of the trip - rent for an extra few days/week just so as you can get to grips with the lenses and get advice before heading off - esp if you happen to rent and big telephotos.