4 season travel Trailers?

If the main plumbing is inside, the rest can be made to work fairly easy. My Pioneer trailer was $10k brand new in 2004 and is by no means a 4 season trailer, but it does well way below freezing. Those kinds of trailers can be had for real cheap, but be careful especially for leaks. Aside from the fresh water pickup and the black/grey drains, all of my plumbing is inside, typically behind counters, etc. I've added ventilation to those areas and rerouted some minor heat ducting to keep them warm. I added heat pads and insulation to the holding tanks and valves too. I use the same plastic membrane that's added to home windows, which helps a fair amount. I can also completely cover the windows with insulated curtains. Insulated covers in the ceiling vents are a big help too. I put plastic window panes on my screen door (like a storm door), which when sandwiched between the door frame and the closed door, makes a great barrier. That one alone made a huge difference in my trailer. I insulated the wall panels that are between the storage spaces and the interior of the trailer. When cold camping I also put insulation behind those storage doors too. Make sure the outside shower is winterized, or better yet, remove the hose portion and stuff that cavity with insulation. After that, make sure you have a generator and lots of propane. Mine is definitely not considered 4 season, but it allows me to go out just about any time I wish. There's ways to make 'em work. You don't absolutely have to have a 4 season, unless you are planning on extended outings. Good luck!