The Fusion will be a good camper, but in Montana, full timing with any camper will be a challenge in the winter months. You'll constantly be struggling with water freeze-up. By the time you invest in the camper, invest in rent for your camping site, invest in the amount of propane it will take to heat the camper in the winter, (and you'll probably be going through 2 - 30 pound tanks a week in the camper, and if it get's colder it will demand even more.) That's approximately $22 - $30 for one tank, so, $44 (for 2 tanks) a week will add up to $176 a month. At $30 a tank, that's $240 a month for heat, and that's if the weather is "with you!" Then you'll have the constant challenge every time you move to underskirt the camper in cold weather, otherwise you'll REALLY experience heat expenses.
Now the summer months will probably be nice, the kids can sleep in a tent giving them more room. Wet dogs in the camper ... (and dogs get wet)... is not pleasant either.
How old are you're kids. If they are young, they'll be as flexible and adaptable as a string of spaghetti in boiling water. But if they are older (approaching teen years ... NO HOUSE is ever big enough!
Here's something to consider.. a mobile home rather than a trailer. They are built to live in, better insulated, they are still mobile, you still have all the issues of preventing water freezing in the winter, but you'll have more room and the cost's will probably be less, because they're made for cold weather.
Otherwise, you'd probably be better off living in a house. RV's are really not a cheep way to live, nor are they simple. They demand constant work and attention, can become very small very quick, and demand a very disciplined life style (everything has to be though out, planned, executed, and maintained. Otherwise you'll go crazy in a camper). It takes discipline, because unlike a house, they become junked up VERY fast if everything is not kept in it's place all the time!
You're idea is noble, but you're setting yourself up for some real challenges. There again, nothing is impossible! People live in trees and eat bugs and live a happy life on the top of some mountain no one' ever heard of and make it too. But you also have to consider, it's not just you, it's your entire family involved.