I'd use a pressure washer and mild detergent but I could be wrong on that too.
I think you are wrong on that. I had to buy a tent this year to replace the one my buddy had pressure-washed... when I mentioned pressure washing to two different tent makers they both shuddered!
Here's a couple phone numbers of tent makers. I'd call them and ask.
Salem Tent and Awning (503) 363-4788
Reliable Tent and Tipi (406) 252-4689
I've done business with both and both are good folks. They will know the answer to your question.
-jeff