The genesis of Itasca is a result of Winnebago's contract with Dodge to exclusively use Dodge chassis's for all Winnebago motorhomes way back when. Winnebago Industries wanted to use different chassis's so they created a different product line to avoid the nasty situation of breaking a contract with Dodge. (This bit of trivia is direct from a Winnebago Industries spokesperson as told to me personally.)
In recent times, Winnebago Industries has positioned the Itasca as an upgraded trim line. The exact differences between the same model year of Itasca vs. Winnebago depends on the model and year. For example, my 2005 Itasca Horizon has hidden cabinet hinges which weren't even available on the Winnebago Vectra. Many items that were options on the Vectra were standard on the Horizon.
All units roll down one of three assembly lines at "Big Bertha" in Forest City, IA. Other than trim, fabric selections, options, maybe colors, they are basically the same.