This is the summary of what we have observed over 3 months of itinerant travel this summer with our T1D teen. In a few words: his BG was better while traveling than during the school year.
Background
When our boy went from a child to a teen (shortly after he turned 12), we saw a radical effect. His time-in-range (70-120) went from 90% to 50%, his SD went from 30 to 50, and he went from no hypos to 15% hypo. During last school year, we were fighting hormonal peaks that would require 15-30 units to bring down, and would carry very long insulin tails. Towards the end of the school year, his peaks were quite difficult to deal with, and the quantity of insulin required was sometimes frightening. We were worried about what would happen during a summer of nonstop travel in Europe, and during the following travel year (we are on the road until Sept 2019).
Travel summary
We left the Midwest on June 8 for Europe, and traveled nonstop until Sept 1, moving in general every 3-7 days, sometimes more often, mostly driving around but sometimes flying.
Overall results
Our fears were groundless (although summer was not without incidents, which I will discuss later). The outcome of summer on his BG was that his average BG went down, his SD went down, and his time in hyper went down as well. Here are the Clarity results.
His BG average went from the high 110s to the high 100s, and his SD went from the low 50s to the high 30s.
His time in range went from the 50s to the 60s, and his hyper time went from the mid 30s to the high 20s, His hypo time remained roughly constant, but with less super hypo time.
I have not entered his TDD into a spreadsheet, but, averaging it by eye, it looks to me that it went down about 20% during summer, before starting to come back up in September when we became stable again (we stopped moving around, and he is going to school in Spain now).
What we saw with BG
To give some context I must explain the type of vacation we took. We were discovering many different parts of Europe: the culture, the nature, the history and the architecture. We would drive long hours from point to point, then walk most of the day for a few days, then drive again. So we were outdoors all the time, and walking a lot.
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My T1D teen’s peaks largely disappeared. When they came, they were much milder, and much less insulin-resistant. I attribute this to the lack of school stress, and to the large amount of daily exercise.
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We ran into more food peaks, due to foreign foods and meals that were difficult to evaluate and dose for. To a degree, they replaced the hormone peaks, but we were able to figure out how to deal with them, and their effect was much less significant than hormone peaks in the end. I will discuss them in a different thread. I did discuss dosing for sandwiches in hot weather here: Travel surprise: dosing for sandwiches in hot weather
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We had to deal with significant basal changes along the summer. The boy drifted back and forth from +5% to -20%, moving reasonably slowly along the range and back, and for a period, to +20% (see below, with heat). He was sick twice. After each bout, he stayed at +20% for a week or so before drifting back down slowly over a couple of weeks. Of course, while being sick his basal was much higher (between +25% and +100%). I will discuss basal drift in more detail in another thread.
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We faced significant highs due to heat and dehydration: his basal went up by 20% during the first few days of heat. We quickly realized that it had to do with dehydration. We had to work surprisingly hard to counteract it. I will discuss this issue in more detail in another thread.
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We had no problems at all with time (longitude) and latitude changes. I discussed it in this thread: Our experience of travel and time changes with insulin basal
In the end, while preparing for this trip was extremely time intensive, in particular to line up logistics while away (we are gone for over a year, and CGM and pump supplies were hard to prep for), the actual travel had less impact than we expected. Even better, it improved his BG rather than worsen it!
So my conclusion is: travel is good for BG 
Go and take a summer off as soon as you can!
Note – this is a follow-up from this thread: