So much depends on your style of RVing and how you visit the places you go.
For us, pretty much anywhere, but specifically for Yellowstone and the Tetons:
First - As others have said I, personally, would not stay in one location to visit both parks.
Second - Being in the Park was key. We didn't care if we had satellite, cable, etc. Our main objectives were to see wildlife, hike, and take the Jeep out to enjoy the scenery. We were out before dawn and back after sunset. We used the RV to eat dinner (sometimes) and sleep. Yes, the sites at Fishing Bridge are tight, but not unbearable and you get to meet lots of your neighbors when you all go outside to watch the herd of buffalo amble through the campground.
Same in the Tetons. We didn't spend enough time hanging out at the RV to care about the amenities that are really important to some people. That's not a dig at any one. Just different strokes for different folks.
Seeing/visiting a place is different for different people. For some, it means seeing the highlights, often by simply to going to an overlook or taking a short stroll. For others (like us) it means finding the back roads that lead to or through interesting scenery, ghost towns, etc. Asking the Rangers for the lesser known hikes that they take their own out-of-town visitors on. Getting far away from crowds to enjoy the quiet side of an area.
If you do want to be in the Parks, make your reservations as soon as you have a date range. The last time we were in Yellowstone, I booked a 2 week period about a year ahead, even though I knew we wouldn't be there that long. We were going to be on a month-long trip and weren't sure of our exact dates. Once we reached Glacier, I called and cancelled the early and late dates we didn't need, keeping only the ones in the middle. At that time, 2011, there was no cancellation fee.
However you decide to do things, I hope you enjoy that area as much as we do. We've been to Yellowstone in 3 different seasons and the Tetons in 2. Still need to get out there in the Spring and see all the baby animals.