Our previous camper was a Phoenix popup. Now we have a Northern Lite 10-2 hardside. The 10-2 is called a four-season camper. But the Phoenix was easier to care for year round. When we returned from a trip, we drained the fresh-water tank. That's all. There was no gray or black tank and no basement.
In freezing weather, we winterize the Northern Lite when it's not in use. This means putting antifreeze in the tanks and lines, draining the hot-water system, outside and inside showers, etc. If we want to use the camper and don't want to de-winterize, we do without fresh water and drain tanks. So far we have not done that.
With the Phoenix, when we traveled in the winter, we filled the fresh-water tank, which was under the sink. On one such trip, the overnight temperature dropped into single digits. Because we had shore power, we used our portable electric heater, not the propane furnace. Nothing inside froze. The five-gallon bucket that stood outside to catch the gray water froze but that was no problem.
What about mpg? With our Phoenix on a 6-cylinder, stick, std cab, longbed Tundra, our best was 18 mpg (on gas), keeping at 65 mph or less. With our Northern Lite on a Dodge Cummins diesel dually automatic, our best mpg was 15.5, driving 55 mph on the interstates. Of course, the two outfits are not comparable. And diesel fuel is a lot more expensive than gasoline.
Other considerations: The Tundra/Phoenix had ideal weight distribution and handled beautifully. The Dodge/NL, not so much.
Climbing into and out of bed in the NL is much easier than with the Phoenix. Also, in the Phoenix, condensation was a problem in cold weather. So far, in the NL, condensation has not been a problem. But we have not camped in very cold weather.
Would we want to go back to the popup? No, but it was fine for us until recently. We are ten years older now than when we got the Phoenix.