NO. Do NOT use air bags to level the TV. While they will level the TV, They will NOT return the lost weight to the front axle.
Now Define "Sags a little". Does the rear sag more than 2 inches, and the front rise more than an inch? If so. That is too much. Or is the front with in a 1/2 inch of the unhooked height, and the rear down less than 2 inches. If so. That is not too bad.
As to As tight as you can get them. How are you going about pulling them up? If you are not using the tongue jack to raise the TT while pulling the bars. They are not tight at all. You can actually raise the rear of the TV higher than it is unhooked. You don't want that though. that IS too tight.
IF you use the tongue jack to raise the TT, AND the TV. That will remove any pressure on the weight bars, and makes them super easy to hook in any link you chose.
So. Add washers, and pull links, Until you get the front back down where it belongs Per your manual, and the rear up to where it belongs. In general. IF you get the front right. The rear will take care of it self. But you must use the jack to be able to easily hook the bars.
Air bags don't help the handling of the TV, and TT in any way.