I read your post when it first came up and been thinking ... really thinking about your question. Here it is 5 1/2 hours later and I'm still debating an answer.
I think, if it's a one time use only, buying is a costly endeavor. In the end, it will cost you a lot more. Add insurance, tags, title, and all the minor fixes you'll be doing (new or used). This is money you'll never get back unless you sell at a profit, which more than likely, will not happen.
If you buy brand new, you'll definitely loose money as soon as you sign the dotted line. You'll never recover that.
For a one-time trip, I think renting is your better option.
However, if you plan on using the camper more than just this one trip, and keep it long term, then buying makes much better sense.
My vote: Rent one.