[masraum]

Have you tried spot cleaning with dish soap, or possibly a mechanics style hand cleaner (preferably with no pumice).

Dish soap is really geared towards cutting grease as is hand cleaner for when you've been working on your car.

My only concern would be what the cleaners might do to the water resistance of the tent. But maybe I'm just thinking about the crappy old canvas tents from when I was in the boyscouts. I haven't been in a tent in many, MANY years.