Carrying a scooter (motorbike, not mobility scooter) ?

400 lbs on the rear of a tt is a lot. I've done 220 with a little 80 cc scoot, and it was very noticeable. remember, you take weight off the tongue as you add it to the rear of a tt, making sway more likely.
A front hitch and carrier on the truck, or putting the bike in the bed would be a preferable choice in most cases.