Arctic Fox is among the best known four-season RVs. I don't know of any other manufacturers who specialize in that area off-hand. Modern RVs are better insulated than older RVs, in the general case, especially when comparing similar quality levels (e.g. a new Airstream or Winnebago will be better insulated and energy efficient than a similar model made 25 years ago...but, 10 year old models are probably comparable to current models, and will often be obtainable for 1/3 the price).

With a travel trailer, there's much less concern about needing to be handy than there would be a with a motorhome. While things are going to break, and things are going to need work, that's true of new or old RVs. My motorhome (which was 7 years old when I bought it) constantly needed minor repairs. Likewise, my 32 year old Avion also constantly needs minor repairs (and it needed and still needs some major repairs).

Sticking with ~10 year old trailers is probably the right time frame for someone who has no desire to have a fixer-upper (I don't recommend fixer-uppers, no matter what, as they always cost more in the end). But, if you start searching well before you want to leave, you can be extremely picky and you'll find someone that has loved their trailer, and kept it in great condition. A lot of trailers only see seasonal use...so they've only been lived in for maybe 6 months, even if they're 10 years old. If the owners winterized them properly, and kept them covered or at least protected from direct sun and such, a ten year old unit can be damned close to new. Higher quality trailers hold up better to aging than cheaper ones, so look for the high end brands (there's too many of those to list, but if you spend a few weeks checking out RV dealers, and RV shows and reading some forums, you'll get a feel for the ones that people really like).