One of the features you will want will be that the unit is perfectly usable with all slides in.
Another really big question is how much do you want to spend, and how big a rig do you want to have?
The more specific you become in your requirements, the better answers you will get.
If you plan on sitting in a park and weathering winter with full skirting, heated tunnels for your waste and water lines, and other full time electric heaters and cold shields, it can be done. If you think that you can just pull in and park, hook up and unhook... I would think again.
The thing to remember always is: Do I have a rescue plan? What if power goes out? What if my water freezes and pipes burst? Your investment can be reduced to zero really fast if you have no alternate means of protecting it from cold weather.
