Effy,
One other thing to consider as you plan, if I may. You mentioned that you will be visiting friends/relatives along the way. In our recent trip to Florida, we spent probably a larger percentage of time visiting family & friends than most would otherwise have preferred. But do we regret it? Not one bit. The landmarks, amusement parks, etc. will be there for years to come, but friends and family can be gone in a moments notice. I'm not trying to bring a "downer" to this, just recommending that you keep emphasis on family connections. Sadly, we know this from experience. But our RV travels allowed us to visit with family members and make trips with those who are no longer with us, including our own daughter. We, of course, included popular sites as well with all of the trips, but we didn't short-change time with relatives.
I know, lots to think about. Also, make sure everyone in the family has a say on what (or who) they want to see along the way. I had our kids & wifey prioritize a list of 10 things they would each like to see on a cross-country trip. Knowing we couldn't see all of them in one trip, but at least it gives us something to work from to keep everyone happy. Places that were mentioned on several lists were given priority for this first trip. Others might have to wait until trip #2.
~Rick