To the OP, no-one knows engineering like the OEM, It takes a gallon of fuel to tow a large travel trailer 10 miles, I don't care what you do, there is only so much energy in 1 gallon of diesel, and there is X amount of energy required to pull the whole rig 10 miles. Your question was "is it worth 3K to achieve more mileage?" First, you have to determine how much more mileage you are DEFINITELY going to achieve. The answer (IMHO) is NONE in all probability. But let's say you get 10% more, that is one mile per gallon more. Somehow I don't think the math is going to work out for you. 25 to 30K pounds going down the road is going to get 10-12 and not much more or less. $3K is not going to change the energy equations involved. You might, however, look at diff ratios for a cheaper more reliable result. If 4X4 perhaps lock outs would be potential energy saving, perhaps tires, both size, and style. You might want to take advantage of lubricants that reduce friction, I might look at the intake system and free up the breathing a little. If you have your oil analyzed you might save on oil changes reducing total operating cost. Anything but spending $3k on an extra (maybe) 1-3 mpg increase or potentially an unknown decrease in MPG.