Welcome!
The
Casita 17' Spirit or Freedom Deluxe checks all of your boxes, and they're fairly plentiful in the used market. Only downside is tongue
weight, which generally runs in the 425# range, give or take. You'll need the AWD w/factory tow package configuration of the Highlander. Check with a dealer to make sure your Highlander has the full 5000/500# rating. You'll probably benefit from a light-duty
weight distributing hitch (Anderson is popular).
Most Casitas do have
air conditioning, and I'd recommend it both for unexpected hot weather and for eventual resale. I'd also recommend the factory
propane furnace for times you don't have shore power.
You can have the same layout in a
Scamp 16' layout 6, except the bed is smaller (45" vs. 54" for the Casita), as are the holding tanks. It would be an easier tow with your Highlander, and no WDH should be needed. It'll work if you only have the 3500/350# Highlander configuration.
Neither is a true cold weather trailer, though. As long as you have heat in the trailer you can use the fresh water and toilet (fresh and black tanks are inside the heated space) but not the shower. Drain lines and grey water tank are underneath and exposed. Single pane
windows and thin insulation means they'll run through quite a but of
propane (and
battery or shore power) to heat.
Bigfoot makes a nice 17.5' four-season trailer, but it's wider, boxier, and heavier than the
Casita and requires a more capable tow vehicle.
Best wishes finding the right trailer for all your needs!