For the best coverage across the country as bad as it sounds T-mobile's "rip-off" service might be the best bet.
http://www.t-mobile.com/hotspot/services_plans.htm has the pricing plans and the $39.95 per month would save quite abit over the day by day charges. ($25 cancellation fee if service is only kept for 2 months) Most hotels offer internet service at a per day rate of $10 but not all hotels have wireless access so you would need a way to accesses a phone line as well. Finding alot of free wifi access spots that happen to be along your wandering trip could be an adventure in itself. I'd let you log on to my wifi network to get email but it doesn't sound like you will be in the Chicagoland area. Happy traveling !