Eat everything / anything.
Get the fire hot enough and it will burn anything.
On a long tour calorie requirements are HUGE, eating enough can get to be a chore. This is where a high fat content really helps, many more calories per gram. But don't neglect the fruits and veggies. Take a multi-vitamin if these are few and far between. I would cook with extra olive oil for things like pasta, and soups, added calories that are good for me.
When in doubt, EAT !
If you find yourself getting irritable, and short on patience, you are probably short on calories. Stop, eat and relax, you'll be fine in no time. Some say honey is the best in this situation.
Milkshakes are magical !