New or used depends on whether you are willing to spend money to avoid personal inconvenience. Do you do your own maintenance or pay someone else to do it. Any RV requires frequent maintenance and occasional repair.
My opinion is you are better off financially buying used if you can do some of your own repairs and all of your own maintenance.
I believe a used RV has had most of the problems worked out but maybe not and that is why it is for sale. It may have a problem that the seller does not want to pay to have fixed.
My personal example was a used RV's refrigerator. In the walk through it seemed to be working fine, however soon after it was a $1500 replacement or some of my time, $100, and considerable inconvenience. BTW $1500 was for a rebuilt replacement installed not the $2500 for a new one.
On the other hand my neighbor and good friend bought a new 5er and has had to take it back for several warranty issues. He lives in it for months at a time at remote work sites and after every trip has some kind of non-warranty repair needed also. Generally, minor stuff but he has already ripped the roof fabric somewhere along the line and needed that fixed at considerable cost (he does not do repairs). He considers his time more valuable than the cost of paying someone else.
My 5er is now 25 yrs old and is fine for occasional fishing trips and a month or two away from home, but I would not want to full time in it. I would not want to full time in any RV for that matter.
So buy used to start and expect to always spend money regardless of new or used.
Biggest new owner mistake in 5ers is buying a truck that is not adequate to carry the trailers pin weight. I would say the second biggest mistake is getting a 5er with 15 inch instead of 16 inch or larger wheels. Third is buying one what is not suited to your expected use (40ft and want to boonedock in forest service camp sites or 25ft and want to live in it all season in a resort).