1. Put a high flow muffler under the truck. Make sure its a trusted name such as flow master, dinomax, magnaflow, etc. Leave the rest of the pipes as long as they are in good condition and just have the new muffler welded in. Most factory mufflers restrict airflow and cause more back pressure than needed. This in turn cause the engine to work harder and run a bit hotter than needed. Causing less horsepower and reduced mileage. Make sure that you avoid any flower master series 40 as these are a bit loud and unless you like the sound of a throaty V8. This minor change will yield minimal increase but when added together with the below suggestions it will help.
2. Change out the paper air filter for a high quality cotton filter. You can order one through AMS oil, or purchase one locally called K and N. These help w/ air flow to the combustion chambers. In doing so you have a more complete ignition of the fuel. Increases horsepower and fuel economy.
3. Make sure the trailer and truck are level with eachother. Put the trailer on the hitch. Stand back and view the angle of the trailer. The truck should not be squatting and the trailer should not be looking like its in a nose dive. Too much weight on the hitch is unsafe and causes for a poor ride in the tow vehicle. Adjust the heigh of the receiver to match what the trailer needs.
The new muffler shouldn't cost more than $100-150 depending on make, shop and say if you want a chrome tip welded to finish off the new job. The air filter shouldn't cost more than $45-60. Total high cost in both $210. What will you see in gain? Maybe 1-2 MPG and a few horses under the hood. Most say they notice a difference. Bottom line, when you read through the hot rod magazines from one to another and talk to most mechanics or those that want to make more power under the hood these are the two least expensive changes made that yield the highest bang for the buck. After these changes get ready to open the pocket book as you spend more money. No need too with the 5.4 as you have a great motor and the truck is more than capable of towing that boat around. If you can't do it with that truck then something else is wrong.