I agree with what Midwest Michael had to say regarding both I-40 and the inadvisability of using Rest Areas and/or Walmarts for your overnight accommodations. There are, however, alternatives to both. I-64 to St. Louis, I-70 to Utah and I-15 from there ends up being only about 65 miles longer than the I-40 route and may take you through some areas you haven't seen before. And rather than using Rest Areas and Walmarts, both of which can be noisy and bright enough to keep you from getting the good nights' sleep that you'll need, you should look at underutilized
state parks near to whichever route you choose.
AZbuck