Hi! Girogirl! Welcome to the forums. Hope you get some more knowlegeable answers than I'm going to give, but I think you will still find excessive cold and snow in a northern crossing during the Spring.
As for maps: Each state usually has a tourist department that could possibly help you with maps and available accomodations.
You might also look into Hostels.
Look back through the archives on this forum and you may find some useful tips and web sites.
Sorry I couldn't be of more assistance.
Good luck on your planned trip.
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ljbike