Hi Kimkalman,
Welcome to the Heartland Owners Forum.
4 season is a relative term, as seasons vary quite a bit in how hot or cold any given area of the country may get. 4 season in central Florida is quite different from Ohio.
Also, there are 2 issues in cold weather: most of the time, RV manufacturers talk about comfort inside, which usually means how much heat the furnace/fireplace/heat strips/heat pumps put out, along with how much heat loss you have. Less discussed is how well the water systems work when temperatures go well below freezing.
It's usually not too hard to maintain a comfortable temperature inside a trailer. But it can get expensive depending on how much insulation you have, how many windows, and how well the slides are sealed, etc. More expensive trailers usually have more insulation and better seals. You'll need an external propane tank - 125 gallon probably - and you'll go through a lot of propane in the winter.
Keeping the water system from freezing is another thing altogether. First of all, you will need tank heaters to keep the holding tank contents from freezing. You'll also want heat tape on the fresh water line from the fresh tank to the pump. Many Heartland trailers offer an option called the YETI package which includes tank heaters and heat tape on the fresh line, along with additional insulation in the front and rear caps. I don't know if Mallard, Wilderness and North Trail offer the Yeti option. If they don't, you would have to have these items installed locally - probably $1-2,000.
There are other things you'll probalby need to do to keep the water running. We have an owner-written
Water Systems Winter Usage Guide that details all these things and will help you in climates where temps go well below 0 (F).