Most folks will advise that you get glass before a new body. Part of this is for simple financial reasons. A good lens tends to hold value better and for longer than a digital body. dSLR bodies, like all digital electronics, tend to depreciate rapidly, even when they're new. And you tend to flip through them at roughly the same rate you'd flip through computers, tablets, or phones. While the 70D is a step up both in sensor tech and physical UI over your T3i, chances are if you wait even a year, the 70D will be quite a bit cheaper. A 10-18 probably won't be any less expensive a year from now.
There is also the fact that lenses are half your camera. And, since many lenses can stay current in the lineup for decades, or only get refreshed every 5-10 years, rather than every year or 18 months, as camera bodies do, the money you spend on lenses simply tends to stay with you longer.
Also, the 70D's big advantages over your T3i are not so much in resolution (that's a minor step up), but in handling, UI controls (e.g., dual-wheel, white balance by Kelvins, etc.), and fast-action capability. Absolutely none of which is essential to landscape shooting. Get the 70D if you want it for its video focusing capabilities, it higher frame rate, and more advanced autofocus system or dual wheel controls. But if you want it to improve your landscape shooting, it's probably not going to be worth the $1000 price tag to you over getting new glass. Landscapes don't generally require fast tracking AF capability.
Nearly any camera body (and many P&S cameras, in fact) can give you great landscape shots. Landscape shooting is sometimes more about shooting technique, support equipment, and post-processing techniques than it is about the specific lens or camera body. Your 18-135, for example, is actually considered wide angle at 18mm (e.g., 28mm equiv.), and stopped down into the f/8-f/16 range is a good performer. f/8 and post-processing are great equalizers among lenses, and can make even cheap glass look good.
I would highly recommend that you consider whether or not there are other areas that might fulfill what you need, vs. an ultrawide or a new body. Consider if learning about panorama techniques to shoot an image in multiple frames and then stitch them together might cover your need for higher resolution and larger scene coverage. Consider whether a tripod might be a more essential piece of gear, so you can use a lower ISO setting and smaller aperture than handholding would let you. Consider whether a shutter remote or cable might be useful for tripod work. And consider whether the software you have is sufficient--maybe spending some cash on specialized post-processing, HDR, panorama stitching, or B&W conversion software might be worthwhile for the style of landscape you want to do. There's more gear to consider here than just lens vs. camera body.
If you do decide to get an ultrawide lens, I'd recommend getting one as soon as possible so you can practice with it before you go on your trip for as long as possible. Ultrawides can be tricky and take time to master. Just cramming more of the scene into the shot isn't necessarily how you compose an effective landscape shot. You need to understand the effects of the distortion such lenses can bring, and how to effectively mitigate some of its possible weaknesses, such as vignetting or LoCA (longitudinal CA, aka "bokeh" CA or "purple fringe"). Learn about horizon placement and distortion as well as how to include foreground interest. I'd also say, given that you're willing to spend $1k on a new body, that you maybe also look at a few of the $600 APS-C ultrawide possibilities, such as the Tokina 11-16/2.8. While for landscape usage, nearly all the ultrawide zooms are good, this is the only crop-body ultrawide zoom that does f/2.8, which can be great for environmental portraits or other indoor shots.