Family Vacation Out West

Regarding redwoods, I would recommend 2 locations. One is the "Avenue of the giants". A web search will give you all you need to know. I would also recommend Jedediah Smith State Park near the coast and Cal/Ore state line. Unbelievable grove of giant Redwoods located across the river from the camping areas.

My one piece of advice to figure what you might want to see in each state is to go to Google and type in " what is there to see in xxx". We have found soooo much info that way. I wait till 1-3 days ahead and make reservations, however we don't travel in busy season (summer). We stayed in Kanab Utah and went to Grand Canyon, north rim, Brice Canyon, and Zion. Note: don't try to see them on the weekends. READ, READ, RESEARCH, RESEARCH. You've got a good start by being on this forum.

3-4 weeks is not long enough to come across country and back a different route.
I would stay off the Interstate highways and use federal US highways and major state routes.
Suggest not making reservations too far in advance. Normally you can find campsites with no reservations except at major parks (not RV) and holiday weekends.
Use a map to start with. Mark the states that you think you want to visit. Go to each states tourism site and order their vacation planner and a state map.
Start marking them up, then have each person mark them up as to what looks interesting to them.