A towed vehicle behind a motorhome isn't going to hamper you in any way in mountains or mileage. With your family you're going to want to see the sights along the way and doing it in a vehicle would be a lot easier than doing it with a motorhome.
As far as your comment about going to Canada and Alaska - I hope you're not considering winter as the season for that. If so, you won't find any RV that will be suitable in -50 degrees. Plan that part of your trip for the summer and you won't need a '4-season RV'. Besides, there really is no such thing. For being comfortable in bitter winter months you would have to do a lot of precautions to your RV - even if the literature says '4-season'. That means four 'mild' seasons. Also, in those kinds of temperatures you won't even find RV parks open for you to stay. How would you get water and dump the tanks?
Plan your trip for north in the summer and south in the winter and you'll be just fine in any kind of RV. Good luck!