Though not bike specific, if you are a AAA member, you may have access to CSAA (California State Auto Association) maps. I find the scale and detail very good for tour planning and on the road use.
Their "Sectional Series" at 9 miles per inch are great for big picture initial planning. As an example the Central California Sectional Series map covers about a 250 X 250 mile area - includes San Franciso well into Nevada.
The "Regional Series" is drawn at 3 miles per inch and really brings out more detail on bicyclist level - the San Francisco Bay Region Map would cover what might be a couple days worth (maybe about 150+ miles) of end-to-end bike travel. Great for on the road navigation.
During planning, I use these maps in conjunction with ideas from Google maps (bike routing, satellite, and street view) and programs like Mapmyride online for distances and elevation changes/gradients. Watch for nearby Adventure Cycling routes (of course).
Oh, the CSAA online services for members includes a route planning tool (car oriented but was very helpful when I tried it for a trip last spring). Rubber band routes on it, it provides turn-by-turn route sheets and maps, and you can have it populate the map during planning with things such as lodging, restaurtants, attractions, etc - all which are clickable to direct links like AAA ratings and related websites. I haven't looked at it for a while but I believe camp facilities were likely included .
I also have looked online for cycling clubs in areas we will be visiting. They often have ride sheets posted which shed some light on whats bike-able, links for resources such as regional bike plans or DOT information, and I've used club contact numbers for help from the locals on getting thumbs-up or thumbs-down on routes I've been considering that may have some element of "questionable-ness".
Wow Neil, after all that I just reread your post and realized your home turf is England (nearly midnight here, working only with half of the remaining functioning part of my brain) - perhaps the above info will still help, or as your plans begin to gel with specific travel areas you can repost and some of us can drum up some maps or extra resources for you. And that reference to AAA - that one was for American Automobile Association! Guess you're not likely a member of that one
Good Luck Tom