Thank you everyone! These comments are all very helpful. I knew that somewhere between "2 days is too short" and "a lifetime isn't enough" there'd be a duration for each park that would let us make see some of the best aspects of each park. I've tabulated these comments and feel like a plan is starting to come together. As to the question about how many times we may be able to go back after this trip, who knows? God willing we'll be back again, but I'm planning as if this may the only time. Which makes all of these comments doubly helpful.
I neglected to mention that Blanding is on our itinerary for Natural Bridges and Hovenweep, as is Mesa Verde (we've been there before) and the Grand Canyon. And we're visiting family in California, and seeing south Texas on the way out, and getting abducted by aliens in Roswell on the way back, and so on. We did the northwest version of this trip last summer and it took us 2-1/2 months. I haven't mapped out this whole trip yet, but it wouldn't surprise me if it took a similar amount of time.
Again, thanks very much. I appreciate everyone taking the time to share their wisdom.
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On edit: My comment about the dogs was mostly in jest. We leave the Weather Channel or Cartoon Network on for them while we're gone (they hate CNN and Fox News), which gives them something to do and keeps them from yapping. We always check with our neighbors when we get back and we've never gotten any indication that the neighbors heard even a bark from the camper. But if that doesn't work, we'll just have them put to sleep. :W