It depends on how thorough the survey is, and whether it covers both house and chassis. If you want a thorough job and the tech has to go to wherever the RV is, 6 hours is not at all unreasonable. Travel time alone will add at least an hour.
You can do a basic check of the house systems in a couple hours plus travel, but in that time you cannot, for example, determine if the fridge really cools (they take hours to chill) and in both LP and electric modes, if the water heater really heats well on gas and electric, etc. Some things just take time.
So, ask the dealer for a checklist of what will be covered, and find out whether you get a report of each item (e.g. a box checked OK or a problem described) or just a verbal "It was OK except for ..." For $600 my expectations would be high.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL