You will probably have a hard choice to make.
When they design a lens for convenience (a wide zoom range like 18-270mm), without making it really large or really expensive, they end up compromising the quality...especially at the ends of the range.
So compared to your 18-135mm lens (a decent, but not great lens) you would probably be disappointed by something like the 18-270mm, or any of the 18-200mm options.
Of course, if will depend on your expectation of quality. I personally don't much like any lens that goes over 3x zoom. (the above mentioned 24-105mm being an exception). So I prefer lenses like 17-40mm, 24-70mm, 70-200mm. Those are the typical ranges where you will see really high quality zooms. But of course, that's not all that convenient for travel.
So your choice will likely have to be between getting an 'all-in-one' lens that is nice for traveling, but compromised on quality...or a better quality lens (or two) that won't be as easy to travel with.