Most of the RV production is in cold parts of the country, and the production season starts in winter. Depending on QC procedures, RVs sent out for delivery may be dry (never had water in them) or may have been winterized after testing the plumbing. I would expect that Airstream checks out plumbing systems wet at the factory, so winterization is necessary before the motorhome goes out the door.
When a dealer dewinterizes is going to depend on climate, sales season, and whether showrooms are indoors. Prep for delivery often doesn't happen until after RVs are sold, except for models brought into indoor showrooms, so even when it warms up stock sitting outside may still be winterized. Not many winter buyers in cold climates will take delivery during winter; prep and delivery will often be scheduled for Spring.
Whether or not prep charges are itemized will depend on dealer policy and factory policy (terms of the dealer's franchise). Some manufacturers include prep as a line item in the MSRP so a dealer for that brand should not be adding the cost to the invoice. Invoice add-ons are often negotiable.
