I am firmly in the buy used camp for a number of reasons. 
Yes, if you are going to keep it a long, long time then buying new to get "exactly" what you want can be doable but..... no one knows how long they are actually going to keep a rig, plans and needs change.  After a fairly short period of time many folks find that their perfect rig is less than perfect and or the itch for something else is there.  if you bought new you take a huge hit on the depreciation.
Some say you need to buy new to protect yourself from defects/problems.  If you buy a quality unit, having thoroughly done your homework this should not be an issue.  If you are worried about the RV of your dreams having problems down the road you are buying the wrong one.  New RV warranties are only as good as the dealer who is close by and willing to stand behind them.  How much help will they be when you are 1500 miles from home?  If you have spent any time on this forum you know that warranties, be they new or extended service plans, are often worth very little in the real world.
Fear of getting a problem unit when buying used is the salesman's favorite scare tactic and it is bull.  A lightly used 1 to 3 year old RV will have had any minor issues addressed and anything major will be glaringly apparent.  If you lack the skill to to do a thorough pre purchase inspection then just hire an expert to do it for you.  It will be the best money you ever spent and probably $300 to $500 depending on what you are buying (obviously motor homes come with a lot more to check vs a TT or fiver).  Additionally most used RV's tend to come with a lot of gear that can be expensive to buy and no, the Dealer probably won't throw it in for free (if they do it will not be high end gear).
Confession is said to be good for the soul, so in the spirt of full disclosure...
I considered buying our Class C new ONLY because there were simply no used ones available (the internet is your friend allowing you to search from cost to coast)).  I was prepared to ignore all of the above just to get exactly what we wanted and eat the rather significant extra cost when lo and behold our dream C popped up up right here in Tucson with 6,205 miles on it.  We bought it for $20,000 less than the new one we were contemplating and could not be happier.
My long winded point being that buying new is easier, usually more fun (at least initially), but far more expensive.  It brings no assurance of quality or absence of future problems.  You can often get exactly what you  want but with a little extra effort you might just find it for a lot less out there in the used marketplace.
As always.... Opinions and YMMV.  Good luck!   :C