At the crest of the Pass north of Raton and at the Colorado/New Mexico border is a RV campground on the east side of I-25. We have never stopped there, but it is there... from those reporting it exists and the Summer temperatures are temperate and pleasant.
I have looked along I-25 from Colorado Springs, Colorado to Las Vegas, New Mexico and the only other option would be an overnight at the several nice Rest Stops in Colorado. North of the Colorado/New Mexico border and before Colorado Springs.
There is a gas station on the east side of I-25 going north into Colorado, where it appears one could exit, find a spot to spend the evening. The station is along the highway outside any city limits. Never noted where, though, but when you see it, you will recognize the 'service station in nowhere'.
There is a State Park southwest of Denver on a reservoir... Chatfield Reservoir, south of 470 and west of I-25. (Littleton, CO/Roxborough Park)
All are easy to... find and also, easy to miss if you are not alert and not using a map for Chatfield Reservoir.