Trying to plan 2-3 hours driving a day, with a few 2 or 3 night stops on the way.
I don't see how you're going to fit a trip from Denver to Yellowstone and back into two weeks, if you're driving 2-3 hours a day, and planning multi-night stops at places on the way, and you mentioned seeing some sights in Montana, Wyoming, and Colorado, as well.
Also, it's a pretty big trip for your first time in an RV. It will probably be cold at night so you'll have to run your furnace and manage your batteries (unless you're staying in a campground with electricity, and Fishing Bridge is the only one in Yellowstone that has electricity, and it'll be $95/night in September 2025), and battery management and charging can flummox even seasoned RVers. You'll also have to be futzing around with water and dumping tanks. And you'll be driving a large vehicle everywhere you go, and of course stowing everything every time you want to go anywhere.
If it were me, I'd fly closer to Yellowstone and rent a car and stay in motels. That way you can concentrate on the sights and not hassle with the overhead of an RV, and especially the overhead experienced by a never-ever.
FYI, here are the rates for campsites; note that only two will be open in the latter part of September (that was our experience when we went in the latter part of September way back in 2011--smaller crowds, but some campgrounds were already closed, so there wasn't the surfeit of campsites we'd expected (although I wouldn't be surprised if everything is now by reservation only)):
Yellowstone campsites September 2025
I don't know anything about any of the lodging there, but the rates don't look that bad at all:
Yellowstone motels September 2025
But I checked for October 2024, and it looks like they're doing dynamic pricing. Sigh. So I assume the rates will be higher as you get closer to September 2025, but I'd recommend reserving any lodging in Yellowstone itself in advance anyway. For the other areas, if you're staying in motels you probably don't need to reserve at all--you said you've done that before and I don't think the pandemic did to motel availability what it did to campground availability.