I love the idea of having a climate-controlled info sign for the dog. Great idea!
My dog is a Belgian with long, thick, and mostly black fur. She doesn't take the heat well either. I think I've covered most of my ideas but I'll reiterate them and offer a few more:
1. Offer LOTS Of water to drink even if this means you have to stop more often for walks and potty breaks.
2. Douse your dog with water to keep her cool as much as possible. And I mean full buckets! Keep a spray bottle in the car to spray her with water if she starts getting dried off before another stop.
3. When leaving her in the car, rush-rush-rush.....make that stop as quick as possible. Heat rises in a car really fast. In fact, I would offer water to drink and then get her fur wet before leaving her in the car. Then do it again when you get back if it's quite hot.
4. Put up a reflective sunshield in your windshield. It wouldn't hurt to rig up some shades for the other windows as well. I have some of those sunscreens for side windows that stick with suction cups for the side windows in my car.
5. Open your windows as much as you feel is safe for your dog so she doesn't jump out.
6. Get a fan that you can set on the dash and leave running to circulate air. Somewhere on the internet I saw a fan that actually fits on the top of the glass of the door's windows. This might be a good way to draw cooler air in the car. But I can't remember where I saw it so you might have to search around if you want to try it out.
Of course, if your truck has a canopy, it might be cooler to leave her in the canopy when you have to run a quick errand without her.
I'm glad you're finding more camping options. Camping is good for you and your dog! The Frommer's guides are good, too. Woodall's also makes a camping guide specific to tent camping sites. I don't think you will go wrong with either one. I have an older Woodall's and a newer Frommer's. I like them both and often take both with me. You might also check out the state parks in the states your traveling through. Also look for BLM land.
The Editor gave you good advice on Crater Lake. Hope you can get into there as it's a beautiful sight!