the high cost of sleeping for free
First, I have to ask what SUV do you have that can comfortably sleep 2 people and gets 34 mpg? I can't even think of any hybrids that claim to do that well.
Even if you think you can sleep 2 people comfortably in the back, I would strongly recommend you scale back on the number of nights you are trying to do it. Remember this, when you are at a truck stop, really all you can do is sleep. You've got really no place to move around outside your car. You really can't set up your camp stove for cooking. They are also well lit, which is important for safety, but also makes it more of a challenge to get a real good night of sleep. Another factor you'll really have to think about is that with 2 people camping, and on the road for a month, you're going to have a good amount of gear with you, and you'll have to find a way to sleep, and still have all of that gear in the truck.
If you were planning just to spend one or two nights at a truck stop, I wouldn't think much of it, but you've got at least a full weeks worth of time you're planning for this. I really think that's going to hurt your ability to enjoy your trip, and for my money, spending $20 on a campsite would be one of the best uses of money you could spend.
If I was going to throw out a number for your food budget, I'd say $50 a day total would be a good ballpark figure.
One final thought on your route. You've got yourself getting home in "3 days-ish." You're looking at a 2000 mile drive and even at a brisk pace, you need to plan for 4 days from Albuquerque. However, I think to plan a trip like this where you are doing a huge loop of the country and then just completely skipping everything on the second half of that loop would be a big shame, and both from what you'll miss, and from the fact that you're going to end your trip on a very tiring and disappointing note.