Welcome aboard the RoadTrip America Forums!
Time is going to be the single most necessary ingredient to make this RoadTrip possible. I look at your two stated goals, "hit all of the lower 48" and "not be in too much of a hurry", and the only reasonable solution involves time measured in months, not days or even weeks. Add in significant parts of Canada, and you're looking at around 15,000 miles of driving (more or less, depending on the particulars of your final routing). At an average of 250 miles/day (my deification of 'not too much of a hurry) and you'll need about 60 days just for the driving portions, let alone time spent sitting in one place, visiting family, exploring some of the more major national parks, just taking a break from the road, etc. So, that's your first hurdle. Can you and the 'kids' set aside that kind of time? Can you all finance such an extended journey? Can the you all be away from work and/or family responsibilities for that long? If the answer to all of those questions is 'Yes', then the rest of it is just details and can be relatively easily worked out. If the answer to any of those questions is 'No', then you'll have to take a step (or two) back and try to work out something considerably less time consuming.
AZBuck