My personal opinion

If you are taking A RORO into NY:

Start in NY City, then Buffalo, zig zag thru the New England States and head to Nova Scotia. After a day or two of rest, swing back to Quebec, Montreal and Toronto, make sure to catch a Baseball game while you are there.

Cross over to Detroit and Chicago (just make sure you are out off both cities by nightfall). Cruise to Keystone National Park and see Mount Rushmore.

After that Head towards Vancouver, just make sure not to fall as sleep while driving thru Montana, also get some Bison breakfast while you there. Stay a night or two in Vancouver. Its British Columbia, so you should feel right at home!!!
After recharging the batteries, take the Old Pacific Highway going thru Seattle, and Portland. There you have two big options: 1. You continue south to San Diego via San Fransisco and Los Angeles. 2. You head towards Boise, Idaho and then to Salk lake city, Utah (the route there covers part of Yellow Stone) and either way you should end up in Las Vegas.

Then head East, loose 15-20 kilos after driving thru Arizona, New Mexico and Texas in the beginning of Summer and take a few days brake in New Orleans.

Once you recover from all the fun, head to Florida and hit Duval Street in Key West, (yeah that's about as South as you going to get) spend a week or two there. After getting the hell of a suntan start miserably heading North to NY and make sure you stop by DC and see the monuments and the White House.
After that If your bikes are still in one piece and running then RORO to wherever you came from.

The Route That I describe its approximately 12000 - 13000 KM. You will of course drive a lot more for the sightseeing part...but 12 Weeks its more that enough time.

Have fun!