I echo Steven's advice. Get the service writers name before you drive in after he promises to honor the price. One thing to know is that the pan that allows the condensation to drain out the tube is not a standard feature on the Dometic. It runs approx $60.00 from a dealer. You may want to ask the dealer if they included it in their quote. The pan could be reused with the same make of AC, but it could also shatter, so the recommendation is to replace it with the AC.
One way to tell if they will try a bait and switch is to pull the inner air distribution box (ADB) and see if the AC is held to the roof with 3/4 bolts. If it is they should honor the price. The ones they do not want to do are the ones form the 70's they were riveted to the roof with 80-100 rivets. They will only do an install if you remove the old one for that price.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato