The Closer to Denver, the less likely a WalMart will be to provide overnight parking.
Several of the cities north of Denver have Ordinances against RV overnight parking.
There is a Cracker Barrel at US-34 and I-25 that has RV slots in the back. Get off I-25 at US 34, West to the next
Loveland RV Resort is at the same exit. They cater to a lot of Traveling Medical personnel working at the Medical Center of the Rockies, though and can fill up pretty fast.
But a lot depends on the route you are taking. If you want to stay along I-80, for example, Holiday RV Park at North Platte, NE is a good, year-round park. Pine Bluffs , at the Wyoming-Nebraska state line has a nice little park with friendly people.
And on I-76 heading towards Denver from I-80, There are a couple of RV parks at Sterling CO, US 6 and I-76
Also Pony Express RV Park at Julesburg, CO