I own or have owned most of them. First, the 5D is an excellent camera and would make a great choice. However, depending on location(s), I sometimes prefer to have two bodies, one with a wide zoom and another with a mid-range zoom, so I do not have to stop and swap lenses. Swapping lenses for a specific shot is not something I regularly do when traveling and rather make do with what is on the camera. I also like a pop-up flash on my travel camera -- helps a lot in those dark rooms or for a face fill in bright outdoor light.
While the 5D and 20D or 30D would make an ideal pair of bodies, the interchangeability of lenses becomes problematic if you need to go wide often. However the 10-22 on a 20D and a 24-105 on a 5D would be pretty ideal -- though then the body I want the flash on when I need it is the 5D...
So, at least for me, a pair of 20/30D's and three lenses is ideal -- 10-22EFS, 17-85 EFS IS (but I am looking into the 17-55 f2.8 EFS IS) and then the 70-300IS, DO or regular, your choice.
There is also rumor of a 50-150 f2.8 EFS IS from Canon this fall -- if so, that would be the ideal tele-zoom for my travel
The final consideration for me is value -- a decent 20D is pretty inexpensive relative to a 5D and as such almost "disposable" by comparison, so I worry not about where or in what conditions I use them.
SIDEBAR NOTE: I have traveled with the 1Ds2. While it produces great images, IMO one must use a tripod and critical focus to realize its full potential. Since travel for me means mostly hand-held shooting with zooms, the resolution gain over the 5D is likely not present in most images and does not warrant the extra weight. In fact, for the same reason, I feel that the image quality gains of the 5D over the 20/30D are minimal at best when shooting zooms hand-held. (And as Ray has pointed in other threads, the smaller pixel-pitch of the 20/30D can be advantageous in certain situations -- like when your longest lens isn't long enough.) Ergonomics, build-quality, viewfinder, menus and features are certainly other valid considerations for using the bigger bodies.
Hope this helps,