What makes a 4 seasons trailer

4 season kind of depends on where you plan to be. 4 Seasons in Michigan means that the furnace and insulation has to be good enough so you and your water lines don't freeze on ZERO degree weather. Dual pane windows would be a must have. All hook ups and related controls need to be enclosed in the heated underbelly. Large propane tanks with a way to get them filled often.

Any trailer could be used 4 seasons if the temp's stay above freezing or below freezing for only a few hours at night. Myrtle Beach comes to mind of a place that any trailer could be 4 season.
Randu