IMO, leaf springs near 15 years old should be replaced. Older the steel, the higher risk of snapping. All my trailers older than 15 years get immediate Leaf Springs replaced. Very low maintenance cost.
If your truck is near this older age and its natural "sagging", I'd replace with new leaf springs as well. Any truck suspension shop will do this task. Since you'll be doing more loading / towing, I might even "up size" as well. re: Add medium length leaf - while getting its rear leaf package replaced. After new leaf spring packs, remember to get alignment as well. re: If rear goes up, then front goes down (which means alignment is also needed).
If Truck's rear continues to sag under load (and still within its weight loading limits), I'd install Timbren SES units next. These are hallow rubber blocks that replace factory bump (bottom out) stops. If wondering, I had Timbren SES (leaf spring version) on my previous Tow Vehicle, and have Timbren SES (coil spring version) on my new Tow Vehicle. Love them so much, my next Tow Vehicle replacement will have Timbren SES units as well.
Hope this helps.