If you load the rear cargo box of the truck with stuff, NOT connect to RV/Trailer and its rear sags, then do buy/install Air bags or Timbrens SES units. I installed Timbren SES units in my previous vehicle and my current vehicle and love them. Especially with their "install them and leave them" design. And, Timbrens cost much less than air bags. With air bags, one must air up/air down and worry about air leaks. Regardless which product you pick, these items level the Truck's REAR (when its rear cargo box is loaded).
To level truck when RV/Trailer IS connected, then buy/install Weight Distribution hitch. The WDH is installed on the trailer's tongue and transfers some of its weight to the front of the Truck. Thus, why its called a Weight Distribution system. For WDH System, ensue one uses the proper size bars as well. For example, if RV/TT's tongue weight is 700 lbs, then get WDH bars in 700-800 range.
If wondering, one may have to get both products. My previous Tow Vehicle has both products and if adjusted properly, they do work great together. But, they do need to be adjusted properly (when combined).