My RV is 37'7" I call it 38 Damon 37.. The last park I was in the site was advertised as 24' but I fit nicely.
I do occasionally, run into issues, for example in 2 weeks I will be in my 2nd choice of RV park because the only site not already reserved at my first choice,,, is also the only site I don't fit in. (Same park where I was last week).
This happens.. Rarley.. At most of the attractions you cited. are going to be like Grand Canyon
At GC I toured the campground that is within the national park.. in a Chevy Lumina APV. and there is no way on God's Green Earth I'm taking my class A there. I am a very skilled driver when it comes to getting in and out of thight places but .. NO WAY Am I even going to THINK about taking the class A there. the 13' Scamp I used to tow behind the APV, yes. but not this big house.
However there were several private parks within a mile of the National park and all of them I'd have fit in no problem. So would you.
Second: IF you are old enough (62 I think) Get the Geezer's pass (Senior pass whatever it is called today) unless you qualify for one of the even better deal passes like Military. This won't help you at the Grand Canyon campground as it is a "Vendor" park, Nor will it help you at the private parks, I've not visited the other national parks you cite so can not comment on them.
But at the park I was in last week (A COE Park) it saved me 13.00/night (I only paid 10 for mine, heard the price whent up though, still 13*7 is 91 and I don't think it went up that far).
And admission to many National parks, like Grand Canyon is so much per car per day.. Without the pass. With the pass you save about 100% on admission (Flash the pass and drive on in) at I think 20 bucks per entry.. Major savings there.