Great minds think alike - LOL
Been thinking about this trip.... and even before you posted the above post, these were my thoughts. Much the same as what you have already discovered. Just that I would not drive the RV across to Orlando. I'd take it back to where you pick it up and avoid paying a one way fee.
If you were to plan the western part as a six to eight week trip the family will have experienced the great feeling of an RV, and you will have covered all of the outdoor attractions which are on most people's 'must see' list. I would then fly east, and do the rest without an RV. The east is nowhere near as RV friendly, and it will be late in the season, which adds more difficulties.
Here's my take, but you decide for yourself. I would fly to Chicago, spend time there. Maybe rent a car to do some sightseeing out of the city. But in the city you will not want a car. It is a hassle.
I would then take the Amtrak to NYC. If it is anything like the train from LA and SF to Chicago, or the train from Seattle to Chicago (all of which I have travelled), it will be a great journey, and another experience for the children. In NYC you will not want a car, nor in DC, nor in Boston (if you are going there.). You could always rent a car in Newark (or elsewhere in NJ, much cheaper than NY) and go to Niagara Falls. It can be done in a few days, but with doing some sightseeing along the way - Boston, Erie Canal, Finger Lakes and the autumn colours, to name a few - it could easily become a week long round trip.
Bus it or train it to DC. Then you can either decide to drive all the way to Orlando or fly, and rent a car in Orlando. Besides Disney there is the Kennedy Space Centre, the Everglades and the Keys. Of course they are only the hilights. You might find though, that by then the kids will be well and truly fed up with being cooped up in a car, and you might spend a lot of your time, just going to the park, by the pool or simply enjoying walks.
Even though you will be staying in hotels and motels, when using a car, it is no reason to always eat out. Carry your own box of cereal, buy some milk, fruit and yoghurt at the supermarket and breakfast is done. A loaf of bread some butter and filling from the supermarket, can make a box full of sandwiches. Disposable plates (as much as I hate them) come in handy at times like that. And even in the evenings, if you have a room with a microwave and fridge (many hotels have these), you can whip up a simple nutricuous meal for the family. A sandwich maker also comes in handy in hotels. So long as you don't have a fire to cook on, hotels will not mind you doing these things. I have never had a problem.
And then you can make the once or twice a week eating out, a specialty. Don't forget that there are also many places where you can buy take away meals, not the fast food type, but nutricious family meals.
Another good thing is to go to a charity shop - Goodwill is the best known, and is everywhere - and buy a handful of cutlery and some cups/mugs. It is usually very cheap, and you can always donate it back again at the end of your trip. I buy all those type of things at Goodwill.
Enjoy the planning, it is so much part of the trip. And see if you can involve the children in it as well. Helps to make it their trip too.
Lifey