You have two choices to get to Grand Rapids, Michigan from Phoenix. Both are approx. 1900 miles - one is all interstates and the other some secondary driving.
Phoenix - if you're close to I-17 take that north to I-40 then east on I-40. If you will be closer to the far east side of the greater Phoenix area, take Hwy 87 to Payson, then 260 east to 377 at Heber, AZ, then 377 to I-40 east. This is a 'cool' drive (temperatures) and o.k. for RVs.
Then for both take I-40 east to Oklahoma City then I-44 to I-55 at St. Louis to I-80 at Joliet, IL to I-94 in Michigan to 196 to Grand Rapids.
The second route - same miles - get to I-40 Arizona as above to Hwy 54 at Tucumcari, New Mexico to Wichita. Then I-35/335 to Kansas City and north to Cameron, MO. Then 36 east to I-55 at Springfield, IL. I-55 to I-80 at Joliet, IL and proceed as above to Grand Rapids.
It's not going to be an interesting drive. There will be plenty of RV parks to stop - it'll be hot so you'll want AC.
We once lived in St. Louis and near Grand Rapids - now in Arizona. Good luck!