I have friends that take Hwy 95 in NV every year between WA and AZ. They've tried all the different routes and it is their preferred route. Ditto TexasShadow that Hwy 58 is fine too. Just note you will go over Tehachapi pass, but no problem there.
I'd look at RVParkReviews.com for Flagstaff. If you don't need hookups and you don't mind driving about 15 miles off I-40, I recommend Bonito Campground at Sunset Crater Nat'l Monument. Or closer in w/o hookups is Fort Tuthill County Park. I've never stayed at any of the private campgrounds in Flagstaff.