If going to Glacier Park I would strongly recommend also going to Waterton Lakes National Park in Canada that is the northern section of this area. There is easier access to parts of Glacier Park by crossing the lake with boats leaving hourly from the docks at Waterton.
For California the Tom Stienstra book "California Camping" is priceless. Great information on scenic areas, campsites by site and what is provided and it is organized geographically instead of alphabetically as is the case with most RV campground guides.
Instead of looking for tourist sites and alternative approach we are starting to take is looking for wildlife areas. Idaho for example has a lot more wildlife than Montana/Yellowstone. If you want to see a moose don't look in Yellowstone but in the wildlife refuges in Idaho.
We do allow hunting in our wildlife refuges and preserves so it is a good idea to know when the hunting seasons start and end in a given location so you can avoid these areas during your travels.
There is great information at the websites of the chambers of commerce for the states and towns and this includes some excellent brochures that can provide a lot of very useful information for trip planning. A lot of their free publications are better than the guide books for sale.
The United States Forest Service also sells maps for each of the national forests and these provide current information as to the roads that are open and camping facilities that are available.
Due to the weather and snow I would start out in April in the southwest and end the trip going through Yellowstone, Waterton, etc. in the June-July timeframe. Glacier NP and Yellowstone have roads that are not going to be open until June.