Replace junk food with a healthier option. Being on the road, it is likely you will have to eat convenience food, but try stopping at a sandwich shop, rather than a burger bar, and ordering a healthy option. Most establishments will be able to tell you what their low-calorie options are. Avoid high-fat extras such as cheese and mayonnaise.
Take a good multivitamin supplement each day. This will help replace some of the vitamins you are missing out on from eating an unbalanced diet.
Eat plenty of fruit. You can pick up common fruits at almost any corner shop or convenience store.
Exercise at least three to fours times a week. You could bring a fold-up bike, some small weights or some training shoes to go for a jog. You could even just go for a brisk walk for 30 minutes. This will help you to feel fitter and more healthy.
Drink less alcohol. Alcohol dehydrates you and will, more often than not, leave you feeling bad in the morning. Having fun on tour is important, as touring is a big part of being in a rock-n-roll band, but try to enjoy alcohol in moderation.