It's not a four season rig, no "C" on the market right now really is. The EKKO is more marketing hype than reality.
That being said, we've used ours late fall/early spring with overnight temps in the -5c range with no issues. The freshwater tank in our model is inside the garage, so some insulation. All of the water lines and drain lines are inside the shell, the hot water heater is an on demand Truma that circulates and has a cold weather factory option kit than can be ordered that protects the hot water components outside of the shell when traveling with the propane shut off . The black and grey holding tanks are exposed but that does not concern me for short periods of cold exposure.
Two concerns for me are the macerator pump and the service bay where all of the valving and fill locations are. I've used a 120v incandescent trouble light inside that compartment when worried and have just recently installed an aftermarket 12v heating pad for a more permanent solution. Still evaluating that. There are ways to use the unit in the winter after it's winterized, and people have retrofit heating pads to the holding tanks and valving, installed window insulation and used the rig year round. The biggest issues IMO are the lack of thermopane windows and condensation.
Two couples have documented their experiences on one of the LTV facebook pages, using the Wonder year round as a ski base in the Rockies. At least one other has documented his cold weather retrofit.