I'll throw my two cent in here, since I am towing with a Frontier with a 4.0 and a six speed manual. I am towing a 19 foot "white box" ( not a fiberglass trailer ). It weighs 3780 pounds on the scale. You will be hard pressed indeed to find a 23 (or thereabouts) feet hard side trailer that weighs under around 5K pounds. Remember that the tow weight
rating of 6300 pounds is really stated for the weight
of a boat. Boats pull easier than big square boxes....so long story short is that pulling a 5K pound travel trailer with a Frontier is probably approaching or exceeding practical limits.
MC1 mentioned an Airstream. For sure, you should include those when doing your homework. The key there from a weight
standpoint is that you'll need to get one ( as MC did ) that's a bit older. Airstreams got heavier in more recent years.
It's really a good idea to go and spend some time sitting in different floorplans to get a feel for whether a trailer feels "open" or "tight" to you. Personally I love my slide out room, and I would have a very hard time buying a trailer without a slide. To each his own on that issue.