At first I was going to suggest that you need to make reservations for places like Glacier, Yellowstone, Zion, etc. While I feel it is probably the wiser thing to do you are going to get advice from some that you can just wing it.
So, you need to first decide if you are the kind of person(s) who are OK making plans on the fly, possibly missing out on places you want to see in lieu of finding alternates that can accommodate you, and making the most of those opportunities.
A 42' toy hauler and large truck are not the easiest things to randomly pull into locations without either reservations or some serious investigation into whether-or-not you will fit. As it is, we've run into issues when changing plans on the fly finding something big enough to fit our 36' rig at locations we want to see. Usually we manage to snag something, occasionally we have to alter plans due to no "room at the inn"; or we have to settle for some less than desirable campground.
We've been traveling since the mid 80's. Most of the time we plan well ahead and make reservations. Sometimes they have gaps we know we can fill in. We tend to travel west a lot (CO, UT, WY, MT AZ, NM NOR, WA, etc.). On our return (back to FL) most of the time we don't make reservations or plans, we just travel day-to-day and plan a day or two ahead making reservations. But we always make reservations well ahead of time since we know what we want to see/do and don't want to miss something we planned on.
I'm not going to make any specific recommendations as to where you should travel or what you should see. But I will tell you that the west is HUGE compared to the east. You'll get it once you get out there. So there are tons and tons of places to see. Heck, after all these years we still go our there almost yearly; sometimes to places we've been, sometimes to new spots we want to explore.
I guess my bottom line is back to whether-or-not you are a day-to-day, take it as it comes kind of couple, or one who would sleep better know your next stops are saved for you.
Enjoy your travels. oh - I'm hoping this is not the trip of a lifetime but the first of many, many more adventures after your retirement.