We have made that trip a half dozen times, but leaving from the west side of michigan (Holland/Zeeland area) and going to either Phoenix or San Diego.
Depending on weather and departure time, we would stop in Springfield MO the first night, then either Amarillo TX, or Albuquerque NM depending on the final destination.
A Walmart in each of the three locations, close enough to the highway exits. otherwise, check some of the RV dealers in the towns you want to stop in. The one down the road from us here in NY, Wilkins RV, has a 3 to 5 site "camp ground" for dealer/garage customer usage. The service manager there had mentioned it to us when i asked about a full brake service before we head to the Keys this fall. We are full time in our WPF and would be out of a home if the rigs brakes needed more than a tweaking. he said "no problem. if it needs parts we dont have, we will overnight order, and put you back together, set you out in our campground". then he proceeded to fill us in on the details. they have it for customers and pass thru customer usage. another option to check out as you travel.
If you like beer and good food, the Springfield Brewery has a wide variety of good beer, along with some great ones. as well as great food and a great atmosphere.
Amarillo has a great dive of a steak house just east of the Cadillac Ranch (Cadillacs buried nose down). I cannot remember the name of it though, Neds or Neals something like that, you know the type, super sized steak you get for free if you eat it all. But this place actually has really good steak. Tender, lots of flavor, and wood fire grilled. Worth the stop, even if you thinks the place is a dump....which you will. there are also a couple of ok RV stops on the west side of town too, Fort Amarillo and Rt66 something.
You will run into a lot of construction this year. the worst of it near St. Louis. they have been replacing highways there for years, and are years from done. If your not in a hurry, and like nostalgia, take Rt 66. its a bitch towing anything of size, but a cool drive with a lot of distractions and places to stay along the way. my wife and i took it home from Mesa back in 12. 5 days of travel at a slow pace, with a 7x14 enclosed trailer towed with our old 01 power stroke. no problems for us, but i sure wouldn't have done that with our new rig, weighing in at 29k. To much stop and go for that nonsense.
hope this spot of insight helped, Have a safe trip.
2012 Ram 3500 DRW G56