Tow/Haul button: what is it for?

I say your sales person was an idiot. If Ford programmed that chassis to have T/H as the default I would think they know what they are doing; I'd leave it on. A MH chassis pretty much is "hauling" all the time. At the weight the typical class C is ready to camp I would think you would want the T/H engaged. Engine braking is important. Transmission life will likely be extended. Fuel mileage on flat terrain may be very very slightly better but I don't think it's worth giving up the improved shifting and automatic downshifting to save a tiny tiny bit of fuel.