Same thing happened to me my first time riding on the hiway...
Best tip I have for you:
GRIP THE TANK FIRMLY WITH YOUR THIGHS!
When it's really windy, you should almost be able to let go (Don't, I'm just saying...) and still feel confident. I remembered this shortly after being violently pushed around (I felt...) by the wind off a transport.
One wind related issue still bothers me, and that's passing trucks going in the same direction. The wind being pushed off the front of them makes me feel as if someone is on the road pushing my tires out from under me. Passing faster helps with this problem, or, if I slightly angle the bike toward the truck before the wind blast I know is coming, also helps.
But, seriously, the thighs on the tank, try it.
