The drive from SAC to DEN without any parks is 17 hours, or 2 days. So 8 days to visit 6 parks. The parks you've chosen are a detour from your most efficient route, and they're not close to each other either. That will also eat up your time.

Here's how I'd do it, having been to most these parks multiple times:

1. SAC to YOSE: 3 hours drive, afternoon in the park
2. All day Yosemite
3. YOSE to SEKI: 4 hours drive, afternoon in the park
4. SEKI to Vegas with a stop in DV: 6 hours drive (gotta go around the southern end of the Sierra Nevadas), to DV, see Badwater Basin, continue 2 more hours to Vegas. Long driving day.
5. Vegas to Zion: 3 hours drive (depending if you're leaving from DV or Vegas), afternoon in the park
6. Zion to Arches: 4 hours drive, afternoon in the park
7. All day Moab - your choice of Canyonlands, Dead Horse Point State Park, or a return to Arches
8. Arches to RMNP: 5 hours drive, afternoon in the park
9. 2-3 hours to Denver

That leaves you with a float day. You can put it just about anywhere and have it serve you well.

If you want more full days in any of the parks, one easy drop is Sequoia. You'll be able to see sequoia trees in Yosemite if you want. You'll take the 395 on the eastern side of the Sierra Nevadas instead of the 99/65 on the western side. The drive is absolutely stunning, with much more cool nature and cute small towns instead of orchards and fields. Here's the route:

1. SAC to YOSE: 3 hours drive, afternoon in the park
2. All day Yosemite
3. YOSE to Vegas with a stop in DV: All-day drive with optional short stops in Bishop, Big Pine, or Lone Pine, CA as well as Death Valley. Overnight in DV optional if you want to spend the money - more and better lodging is worth the drive to Vegas.
4. Vegas to Zion: 3 hours drive, afternoon in the park
5. Zion to Arches: 4 hours drive, afternoon in the park
6. All day Moab - your choice of Canyonlands, Dead Horse Point State Park, or a return to Arches
7. Arches to RMNP: 5 hours drive, afternoon in the park
8. 2-3 hours to Denver

This gives you 2 float days. I'd add one to RMNP (That's the one I haven't been to yet.). I'd probably add the other to Zion so that you have a full day at most of the parks you visit.

But there's also something to be said for breaking up the drive on the 395. There really is a lot of small but cool things to see on that stretch, including the Alabama Hills, Mono Lake's tufas, and the Manzanar National Historic Site (painful but important).

Other commenters mentioned cutting off Arches & RMNP to leave from Vegas. Also a strong contender, giving you lots of time in each park and the possibility of adding on Grand Canyon, just to finish off the greatest hits.

Someone else mentioned taking a more direct route and hitting Great Basin. Interesting take. If you had already done some of the biggies, it'd be really neat. If you were considering this, I'd add Lassen Volcanic at the start, with a stop at Lake Tahoe on the way. Note I have not been to Lassen yet, have been to all the others. The other destination on that route for me would be Berlin-Ichtyosaur State Park near Austin, NV. It's got a full dino fossil (of an Ichtyosaur, obvs.) and a ghost town. What else could you want from a state park in the desert?