Two other supported options:

http://www.abbike.com/

http://cycleamerica.com/

Personally, I did not want to go supported because the days are typically longer. I had no time restriction. The only reason I did a group tour was that I had zero loaded touring or camping experience.

I am glad I went the route I did. I used the experience and confidence I gained to do two long, solo tours the follwing year.