New springs or air bags?

My '15 OR Prerunner is totally stock. From some angles, it looks like the rear leaf springs are just starting go to a negative arch. From other angles, they just look flat, which I guess is pretty typical. I measure 21.5" from the center of the rear hub to the bottom edge of the fender. I tow a small camper with about 160-180 lbs tongue weight and 300 lbs or so in the bed. My wife had me haul two bed loads of fill dirt a few months ago which I'm pretty sure fatigued the springs some. The rear seems like it is sagging a little bit more than when I bought it last January. Not a lot. I've been thinking of installing Firestone air bags for a while. Now, I'm wondering if it might be time to replace the springs. Should I replace the rear leaf springs, or just go ahead with the air bag install? I'd probably go with the General Springs HD packs if I went that route, but I'd have to hold off on the air bags for a while. I'm planning on keeping the truck pretty much stock.