Iowa ain't too bad as long as you stick to the highways. the interstates in iowa suck but if hitch from a good Exit you can get a ride the whole way across.
My friend always hits the truck stops and asks semi truckers for rides. Note he usually travels alone with about zero gear (clothes on his back, a hobo tool, sharpie, and bic lighter). But he says he always flys through Iowa like lightning.
florida's the worst ive had, but ive only done the east coast yet
He also recommends flying a sign with a basic drawing of a truck and an arrow pointed at the bed of it. Got him up and down the FL coast, and me across MO after rockin a sign for under 10 min twice.