Hey - it's your anniversary and many folks don't make it to 20 years anymore. This is special!! If she wants to see the Arch I'd recommend routing yourself that direction and spending the night at this RV park directly across from it. It is an awesome view at night....no need to go up in it - all you'll see are buildings of this big city. :)
http://www.goodsamclub.com/travel/campgroundsandrvparks/generalinfo.aspx?cgid=990000544I agree that spending as much time as you have for a national parks is much better than not going at all. If you can both drive it will be a big help.
If you haven't already, check out Yellowstone's web site. It has lots of good information on things to do, road information, etc.
https://www.nps.gov/yell/planyourvisit/index.htmIf you can route yourself to Mount Rushmore, definitely spend a night in the area and do it if you think you'll be stopping for a night around there anyway.
Have a great trip!