I am assuming you are looking for something that will take 20 degree nights, not zero degree nights, because you say northwest. The things that make a trailer 4 season weigh a significant amount - insulated tanks, ducted furnace heat into the tank area, dual pane windows, more wall and ceiling insulation. I have never seen any of those things in a 2000 pound package. Do you just want something to spend a weekend on occasion? If so, you can winterize your trailer and then use only water jugs (not the trailer's tanks) and flush the toilet with RV antifreeze. You will still be running your furnace non-stop because of limited insulation, but it would work for a limited time. That is probably the extent of your choices in that lightweight a trailer.
BTW, we used to camp with a pop up in the Rocky Mountains during shoulder seasons when temps were down to 25 at night but warmed up during the day. It was not a comfortable thing, but we were experienced tent campers, loved the season and area, and knew the drill. Not something I would recommend for new campers.