West of the Mississippi, it would be relatively easy to do a ride that would follow only dirt roads with minimal pavement. The Great Plains is a grid system of roads and you'd only have to go a mile or so away from any paved highway to find a dirt road. You could probably follow the Lincoln Highway corridor and be reasonably sure you'd have access to services on fairly regular intervals.

The problem I see with the TAT for motorcycles is that there are long stretches in the west (from Arkansas west) that pass through some very sparsely populated areas. Finding water in the Oklahoma Panhandle would be problematic not to mention Utah and Nevada.