Hi Wkdlizzy,
I did this trip last year in May and can promise you that it's absolutely brilliant. The national parks scenery is just incredible; add to that stories of the old west at Deadwood, the privilege of visiting the Little Big Horn battlefield, the magnificent Monument Valley (so familiar from the old John Wayne westerns), the indescribable Grand Canyon (do the helicopter flight if you can) and the bright lights of Vegas, it's a holiday I will remember for ever.
You will have a fantastic time; just take plenty of photos to remember it all by!
All the best
Mike