Welcome to The Forum! And good luck in your search. I've only purchased new
trailers and have no experience with used ones. Although, I'd have to say I
wouldn't have purchased the 10-year old trailer I traded in on my Eagle. My Eagle 266RKS is a rear kitchen travel trailer, and I love it! Only has the one slide, but it's perfect for just me and the dog. I, too, will be full timing (in two weeks) and found the rear kitchen to be more open-feeling. Everything is easily accessible, they have a lot more countertop space than the typical mid-kitchen models, and they generally have more wardrobe storage, as well. I, personally, dislike a second slide-out if it's on the starboard (door) side, because it cuts down on your patio space. But that's a personal bug-a-boo of mine, and may not be of concern to you.
Some folks who owned older (10-15 years) rear kitchen trailers & 5'ers complained about the jostling their kitchens took, being behind the axels. I've heard their pots, pans & dishes would end up on the floor by the time they made it to their destinations. I've also heard that problem has all but been eliminated by the wide-spread axels and better suspension systems on todays trailers. I've only made one trip in mine - almost all freeways - and didn't have a single problem.
Getting back to new vs. used: In your situation, I might suggest a gently used 5th wheel as sort of a "Guiney Pig". Maybe a 2-4 year old with a floor plan similar to what you might want for your "retirement home-on-wheels". Just like cars, the biggest depreciation hit comes in the first year, so you might find one at a pretty good price. You can learn about the camping lifestyle, and what you do and don't like about the make, model, floor plan, etc. Then, when you're ready to retire, you can upgrade to the "home of your dreams". Just don't get something too much older than that, and plan on keeping it for 10 years. While there are exceptions, 10 years is about the average life of a trailer/5th wheel. You'll want to have something left to trade in on your new one. It doesn't seem to matter if it gets used a lot, or sets in storage. A rig can rot in a storage lot or the back yard as easily as it can on the road. And, in ten or so years, if you like the lifestyle and do decide to upgrade to a newer, nicer 5'er, you'll probable be ready for a new tow vehicle, as well.
That's JMHO, for what it's worth. Hope you enjoy your search, and will post some pics when you make your decision and bring home your baby. Best of luck!