I've used the auto purchase program, and I keep using the discount on dealer service Costco offers.
You can probably check with Costco corporate to verify their RV deal has a "best price" clause. If so, if a dealer really has a lower web price than the Costco offer they won't be in the program for long.
Bottom line is auto dealers have had "internet specialists" for years, frequently the same person is their fleet person. It really can take a lot of the time and trouble out of a transaction on a new vehicle. The better deal is buying pre-owned if you can find what you like.