Just follow google maps is my suggestion.
400-500 miles will put you in Albuquerque or Santa Rosa on the first night and around OK City the second. There are lots of options for camping around Albu. I like Isleta Lakes
link and Santa Rosa Lake state park
link but both of those are 5-10 miles off of I-40.
In a motorhome I would use truck stops/Wal Mart and skip trying to find campgrounds, since you seem to be in a hurry.
I try not to go east of the Pecos River in the RV, so I am sure others will have suggestions for that piece of your trip.