We just did that route going the other way last month. (Wondering if you are headed for the Vermont Rally?)
We highly recommend taking the ferry across Lake Champlain, providing magnificent views. They easily accommodate motorhomes.
If you stop at Niagara Falls be sure to spend some time on the Canadian side. It is vibrant with a healthy economy. The American side is going through a very depressed economy, but you might find some enticing campground rates on the American side. Things are pretty run down there however. We took the Interstates using the Northway out of the Adirondacks and the NY Thruway coming west. You could reverse this on your way east. Then exit the Northway to Port Henry and take the ferry across to Burlington.
If you go the Canadian route, come down through the Grand Isles (North and South Hero) and take the South hero ferry over to Vermont.
I am not sure where the rally is, but if it is near route 20 we far prefer that route through Vermont versus the Interstate. Not as fast but exceedingly scenic, passing through really nice small towns in Vermont. September should be a great time of year for this, but do not linger into October. <g>
