first, I really doubt the tent is toxic. Sun Huts had that problem a few years ago, but I would believe that is no longer an issue with any real tent.
If you gave a good flushing, with PH adjusted water and the environment is good, you should see an improvement (in 2-3 days), almost universally. (minus under feeding, and thats pretty god damn rare, who under feeds?) and you have not seen any improvements I would start feeding again, with half strength.
The negative pressure issue is real, the question is; is that whats causing this plant issue. Negative atmospheric pressure issue would cause the stomas to gradually get worse since the plants stomas are going to gradually close, they don't all shut at once. Maybe get a voltage regulator and slow down the pull from your fan to a little less and see if they improve but don't forget your security concerns those are real too.
if the fan has a big draw, with a lot of pull, that would be my guess.
How long did you veg for? (Im looking for the real start date of the issue), and did you do any fan upgrades, or room upgrades (more air tight) in between?
good luck =)
p.s. if I got this shit right, negative atmospheric pressure, from a help me post on the internet, I should get some kind of fucking award huh? Lol