I would think that the worst time would be when the manufacturer was changing between model years, particularly if there is a big change. That would make the worst builds (depending on manufacturer) in July and August.
I don't think the manufacturer sends particular units to an RV show; it is the dealers at the show that choose from their stock. We have run into quite a few dealers at RV shows who don't fix anything but promise to "fix it if you buy it." We avoid those dealers. We have run into broken trim, broken slides, ripped floors... and they still sit on the showroom floor. Some dealers just won't put any effort into showing their products at their best. Other dealers do. We shop there.