My main questions to those who are familiar with the areas I mentioned are:
1. Which is the BEST route to take that have the LEAST amount of hills or mountain pass (freeway to USE or AVOID), between places I listed?
2. Any areas of interest between each stops, or nearby our stops?
3. Recommended RV campgrounds at the points of interest?
1. "Best" is always subjective, but you've qualified that with the least amount of hills or mountain passes. With that in mind, at least for the first leg of your trip, from Page AZ to Denver or Fort Collins CO, I would suggest this: Take AZ 98 to US 160, and then take US 160 all the way east to I-25. That takes you through Cortez and Durango, where there's lots to see and do (Mesa Verde NP, the Silverton-Durango Train, among other things) and on to Pagosa Springs. This route you'll have Wolf Creek Pass to go over, and while it's steep on the west side/headed eastbound, it's also very doable and RVs/trailers do it all the time. This route on US 160 will take you across the San Luis Valley through Alamosa (Great Sand Dunes NP) and over North La Veta Pass, which is not much more than just a hill, not a challenge at all. You'll get to Walsenburg with a minimum of mountain driving in Colorado, and then can take I-25 north to Denver area.
As an alternative, you could also take the same route, but at Pagosa Springs, take US 84 south into New Mexico to Chama and then take US 84 south to US 64 and take that east to US 285 where you can head north and hit Alamosa, avoiding Wolf Creek Pass. It's going to be hilly in places but no high mountain passes like Wolf Creek.
Alternatively, you could also take the same route, but in Chama NM you could do the Cumbres and Toltec Railroad, which is an incredible excursion and one that really should not be missed. You could also take NM 17 north out of Chama to CO 17, which will take you over Cumbres and La Manga Passes, but these are not difficult passes to cross, and it's a beautiful drive.
There's other routes as well, that I'm sure others will suggest. These suggestions meet your criteria for the least amount of mountain passes/hills, but you're going to have some mountains to cross in some form.
2. As noted, there's Mesa Verde and Durango-Silverton Train in the 4 Corners area (and so much more). In Alamosa area, the Great Sand Dunes are an impressive sight and can be seen for miles around as you drop down into the San Luis Valley. There's the Cumbres and Toltec Railroad that you could take either at Chama NM or Antonito CO. There's way more to see and do in the southwest part of Colorado.
3. There's all sorts of campgrounds along the routes noted above, both commercial, USFS and National Park. Depends on your specific needs.
RVParkReviews is a good starting point to scope out specifics.
What I've given you here is one very small tip of one very large iceberg, just in the first leg of your trip. And this doesn't even get you to Denver or Fort Collins, where there's also lots to see and do in each city, not even counting trying to get up into Rocky Mountain National Park.
There's a lot to do just in this first leg, unless you're interested in simply getting to Denver/Fort Collins as quick as possible as a stopover, and continuing on to subsequent legs. Perhaps more information as to what you want to accomplish in each leg of your trip, would be helpful for us to hone in on some specific recommendations.
2007 GMC Sierra SLE 3500HD Dually
2016 Coachmen Catalina Legacy Edition 243RBS
2007 Keystone Outback 25RSS - R.I.P.