We went to AK from NC in 2010 and left on May 30th and returned on Sept 1st.
We took the Northern route through Iowa, Minnesota, North Dakota and Montana on route 2 which is a very nice drive. We then entered Canada through St Mary which is on the Eastern side of Glacier Nat'l park and headed for Calgary and Lake Louise, etc.
Your only problem is traveling on Rt-80 from Pa whuich I hate plus in Illinois it is a toll road. That road is not good and the trucks beat it up.
As far as the return trip I wouldn't leave any later than the middle or end of September. Also I would think you can see all there is too see in 3 months with no problem. I did it and even had time to spend to do some fishing and even take 2 boat trips.
Enjoy your trip as it is the trip of a lifetime. If you have any other questions PM me.
2014 Winnebago Adventurer 35P,Ford F-53, V-10
2011 Ford Escape,2000 Roadmaster Tow Dolly
"Have a Great Day, Enjoy RVing."