suspension question

I have a 04 double cab TRD with 200k miles with the stock suspension. Im looking to upgrade the suspension (maybe a small lift of 3" or so, or keep it at stock height, whatever helps my weight problem) but my main complaint is when i go camping the bed sags alot. I have a camper shell that always stays on. So im looking for a set up that allows me to fill up the bed (with camper shell on) along with my 8' trailer filled and not have it sag so much. I realize it will sag with all that weight but its really bad when i have that much weight on it the way it is now. I have a large family so we bring tons of crap with us. I want to upgrade before this summers camping season. I also have a 18' boat that gets used every now and then. It sags with that sucker on too.

Any help or suggestions on what I should do or brands I should go with is much appreciated. Im open to budget fixes along with quality expensive set ups. I want to know all my options. Thanks!

 

Someone can correct me if I'm wrong but I believe a set of airbag springs would be the best option