I would agree with Rayman. It will be safer to tow the trailer with a WD hitch setup. I have 1,000 lb WD bars, but my TT weighs in at 5,620 when loaded to go camping. You would probably be fine with the 750 lb bars. When set up correctly, your truck and trailer will sit nice and level.
I would also suggest finding a place to have it weighed when it is fully loaded, just so you know what it does
actually weigh. Check your local yellow pages for public scales. Even if you do have to pay to have it weighed, it should only cost @ $10 or so.
Yeah, I would probably ditch the K&N FIPK and go back to the stock air box, etc. I have made a couple of mod's to help with towing. The first and least expensive was the addition of a Hellwig anti-sway bar for the rear. Then a couple of years later I installed a set of the JBA headers and finally the Unichip PnP. All of these work very well together for the trailer I pull.