A few thoughts:
1. You don't have to cook/refrigerate everything. Eat a lot of breads, peanut butter, canned fish, nuts, raw vegetables and fruits, crackers, rusks, etc. I hitched around the US for a month, years ago, and ate no cooked food except one splurge wedge of pizza. It was fine (it was summer, too, which may have helped). I came home healthier than I left.
2. You don't have to eat everything in your hotel room. Relax a little and get something warm and/or meaty from a restaurant (maybe an "ethnic" cheap one) every day or two. And/or hit a buffet for lunch, then have very small meals the rest of the day.
3. Consider travelling with an electric kettle instead of a hot plate. It's lighter and less "hey, I'm cooking in my room!"
4. If you're frequently staying at the same hotel, once they know you ask if you can leave a small suitcase full of food and food prep items behind the desk somewhere.
5. Cook salmon in your room:
http://www.youtube.com/watch?v=0XwBhzQbIvw... I actually do it differently, by putting the salmon in a ziploc, removing the air, and popping it in a sink full of hot tap water for five minutes or so (you can refresh the hot water partway through if you want).
6. Consider travelling with a tiny food-improvement kit -- olive oil, salt, pepper, sugar, single-serving mayo or soy sauce, garam masala or chili spice, dried basil, etc. This lets you turn plain stuff into something that feels more like a meal.
EDIT 7. Get a thermos and bring your own tea/coffee with you for the day. That's one of the biggest markup items that a per diem covers.