6 weeks means you have a very, very ambitious itinerary! Favorites/must-sees/definitely-avoids will depend on personal taste and experiences. With your timeframe, you will not have enough time to do more than just sample some of the highlights, so I hope you have the option to take future long trips and revisit your own favorites in more depth!
My personal opinion would be to avoid any and all big cities as much as possible. :) If you need to Wallydock, stay in the suburbs. Sounds like you would enjoy wilderness and nature, so don't waste your very short time on cities.
I would also skip Niagara, but if you haven't ever been there, or if it's special to you, then keep it. Since you'll be following a northerly course, and if you have valid passports, take them with you and go across Canada to Michigan and spend a couple days around Mackinac Island. (Take a ferry from Mackinaw City or St. Ignace to the island for a day trip back into time.)
Take interstates whenever possible to save time. The major parks you mentioned are must-sees from most people's points of view, and if there is time, you can visit at least some of the southern Utah parks in conjunction with the Grand Canyon. Be sure to allow at least one full day at each national park, and some of them - Yellowstone in particular - will require a minimum of two or three days each in order just to hit the highlights. Grand Teton is next door to Yellowstone - include it with your Yellowstone visit.
Remember that some of the lodging - including RV spots - in and around the more popular parks have been booked up for months already. I strongly recommend reservations, and absolutely they are a MUST for the 4th of July weekend. Other than Glacier, don't worry about snow on the roads. Yellowstone's roads are all clear now except for the NE entrance, which you probably won't have time for anyway. You won't be able to fully appreciate Glacier until Going-to-the-Sun Road is open - and you can't cross that with your RV due to size restrictions. (Take the Red Bus tour - worth the money!)
Good luck!