Thanks and a follow up question

Posted this as a PM to your inquiry, will post here as well to share with others, as a potential route. This is the way I would go.
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US 36 from Estes Park to Lyons. When you get to Lyons, you want to take CO 66 instead of US 36. CO 66 will take you straight out to I-25 on the north edge of Longmont. A LOT less traffic that way, it's a good, wide 2 lane from Lyons to Longmont with some 4 lane around Longmont.

Take I-25 south and watch for the exit for I-76 (it will say Arvada on the sign). It's a sneaky exit so watch for it and get over in the far right lane as early as you can. There will be an exit for US 36 to Boulder, immediately followed by the exit for I-76/Arvada (I believe one of the signs also says Grand Junction).

Stay on I-76 to the very end, it will merge with I-70. Stay in the 2nd from the right lane if you can, or even the 3rd. You want to stay on I-70 toward and past Golden. The highway starts to climb some, just a gradual climb. Past the Golden exits you'll next come to the interchange for CO 470 (or it may say C-470. Same thing) Get in one of the right 2 lanes (both exit), there's a loop curve, and then you'll be on CO 470. From that point, you'll be going back downhill, and it will take you all the way to the exit for Chatfield SP, which is CO 121/S Wadsworth Blvd. Exit and turn right, it will take you directly to the park entry, on your left. Past the entrance station, you'll make a right and take the Perimeter Road south around the end of the lake, to the campground area. There are 4 loops and they go D/C/B/A as you approach the campground area.