I am an adult onset Type 1 person. I have hiked/backpacked the last 4 years with it. Last year I did the JMT and in a few days the TRT.
The main thing that I changed was to avoid the sugary and high glycemic index foods. Thus things like Pop Tarts, instant oatmeal (which is full of pure sugar), GU, Gatorade (high dextrose corn syrup) and rice. Instead I select foods that have complex carbohydates that break down slower and have less insulin spiking. As many others have mentioned - lentils, quinoa, pasta, couscous, chickpeas (garbonzo beans), top ramen, soba noodles, tomatoes, dark chocolate (in moderation), peanut butter, granola, bulghar wheat, blueberries, cheese, salami, Rye Krisp Crackers and so forth are in my diet.
I do not recommend you doing a high protein only diet on the trail as you do need the carbohydrates and fat for energy. Of course high protein is needed to repair/rebuild your muscles.
The climber diet of "shooting GU" every 45 minutes and high sugar works great for a day trip - but it wreaks havoc on us with insulin issues.
Yes Pop Tarts are the universal trail food - but they really have no nutritional value and cause bonking for me.
I may be lucky in that I don't require alot of insulin as my pancreas still produces some (just not enough!). I have found that if I study the trail, I can adjust my insulin needs based on my physical exhertion. So if the trail calls for a long uphill section, I know that I will be burning out lots of the sugars I have taken in and thus able to reduce or better calculate my insulin needs for a particular meal. Another method is to nibble as you go - thus you are burning up your food intake as you go and you sugar levels are not spiking and thus needing insulin.
I am an avid FBC user (a Sarah/Laurie groupie or convert?) and I just avoid the recipes with instant rice and substitute couscous in those recipes to reduce the insulin demands that rice has.
Don't worry about checking your blood sugar all the time, after a while you will get the knack of how much insulin to take based on what the trail ahead requires. I check mine before each meal or if I feel sluggish or light headed (not from the altitude..) - if it freeks people out - that is their problem. Getting a proper diet on the trail of good foods will actually make you a stronger hiker than people who abuse their body with trail junk food. The insulin you take actually increases the absorption of vitamins and minerals into your body as it is more highly concentrated that your natural insulin. So reducing the garbage in is a good thing.
As far as lows - yes I will occasionally have them and I will sometimes take some GU to get it back up to normal. Also my doctor says don't worry about your sugar level running a little high (120 to 150) as you will be burning it out. Also keep in mind you have an exercise hangover after hiking for the day and thus you may need to reduce your insulin needs at night. So you may want to experiment taking less Lantus (background insulin) than normal as that is what keeps you regulated between the quick action Humalog. This is what my doctor recommended and it works pretty well for me.
Take care and just remember - while you have diabetes - it does not have you and may make you a healthier person in the long run than people who abuse their bodies with junk food.