Air filter, add some fresh grease to all your fittings, check the condition of hoses, belts ,brakes, check for excessive play in steering components, look at the inside of your tires for bulges or cuts. I am assuming that you have already checked the outside and tread areas. Look the frame over good for damage, like cracked welds and such. Look at all nuts and bolts, if you see a shiny area or clean area in the middle of a bunch of dirt around nuts or bolts, something is loose. Lower and raise spare tire several times to make sure all is working smoothly, apply a spray grease where necessary, check the spare for damage as well as air pressure. Make sure your jack is working properly and all of the necessary hardware is there. Lift each wheel independently and check for any play, grasp the wheel at the twelve and six o'clock position and push and pull, each hand in an opposite direction and then repeat at the three and nine position, listen for grinding sounds as you spin the wheel, you may need to lift the whole axle depending on what you have in the pumpkins, to do this part. Run some motor flush through when you change your oil. After you drain your oil, (put the plug back, accidents happen) and before you change your filter add six quarts of a high detergent, low cost, oil 20w-50. Start your motor, count to three and cut it off. Wait a few minutes and repeat that three or four times and then drain completely. Remove oil filter while the drain plug is out. Fill the filter with recommended oil, Castrol makes a good diesel oil. Replace filter and plug and filler up. I flush mine like this every forth oil change. With over a quarter of a million miles on my 7.3, my oil still comes out looking almost new. Performance Formula Stanadyne Diesel Fuel Additive is also a must if you want to keep your fuel system in tip top shape. No, I do not work for Stanadyne or Castrol, I just know what has been keeping my 7.3 running at it's best. Now, If I could find something that good for electrical systems, I would be set.
I hope this helps!