I think the blanket is such a lovely idea - very from the heart and I think a family that takes exchange students would love it! As to teens, I haven't a clue.
You should be able to eat okay (if not grass-fed/wild) at grocery stores along the way or at diners, you should be able to get salads and plain cuts of meat or eggs. Signs along the road that read: Food/Gas/Lodging are often good for more variety, though that's not a given - you might get off the road only to find one motel, one gas station, and one mini mart or McD. Often you can tell by the names on the sign if it's a busy or one-horse stop.
If you spend any time in Seattle, and I hope you do since it's a gorgeous city, check out Pike Place Market - a little touristy, but also a fantastic fresh food market.
My last road trip was ~2000 miles, and I did well with a cooler - not sure what you call these in NZ. Can be as inexpensive as a styrofoam box with a lid or as pricey as insulated with wheels and a handle. Put in some ice (can be purchased at almost any gas station) and it will keep your primal food pretty well. I stocked up on dryer meats like salami, french cheeses, olives, pickles, etc., and avoided a lot of fast food.
Not sure if any of that helps, but I'm delighted that you're adventuring by road - it's a great way to see the States. Vancouver to Seattle, I'd ask your travel agent. If not, there should be a Visitors' Bureau in Vancouver that can help you.
I so hope you have a blast!