Gauges first, before doing anything to the engine. Typical aftermarket gauge recommendations for a 24v truck with auto transmission are boost, exhaust temp, fuel pressure and trans fluid temp. I knew a guy who got extra senders for his transmission temp guage, putting them in his differentials and transfer case, wiring them all up to a multi position rotary switch with some good labeling. It allowed him to select what fluid temp he was looking at. That's pretty overkill, but it looked really cool the way he did it.

For the brakes, get the bigger wheel cylinders for the rear drum brakes. Do some searches here--you should be able to come up with a NAPA part number pretty quickly. WAY better braking. If you like you can also play with the proportioning valve at the rear axle. Some folks have modified the pivot arm, adding a turnbuckle for easy adjustment. This allows you to dial in more rear braking when you have the trailer hooked up and more weight on the rear axle, or you can dial it back for when you're empty.

Get a set of starter contacts from LarryB: Dodge Cummins & Diesel Parts, Starters, Solenoids: LarryB's LarryB's These large contacts last longer since they have a lot more surface area. Super easy to change out too, and they're the #1 reason the starter motor fails. Most folks have no clue about this and spend a lot more money buying a new starter every 50k - 100k miles or so.

Since you're towing, you might also want to look at an exhaust brake. When I put a Pacbrake on my 2000 truck, I realized it was hands-down the best mod I ever did. At the time, I only had a 2000 lb camper in the bed, but for long, winding downgrades you simply can't beat it. Your normal service brakes will last way longer.

Lighting is pretty poor on the non-sport trucks, so if that's what you have it's pretty simple to upgrade to the better headlight housings used in the sport trucks. I believe these typically use an aftermarket wiring harness that also adds relays, taking the current load off the headlight switch, which is a really good thing. While I don't have the sport headlights on my 2000 truck, I did add an aftermarket harness with the relays. It helped a bit with headlight brightness because the new harness uses larger gauge wiring, resulting in better voltage at the bulbs.

For comfort, you might want to look at swapping the driver's side and passenger side seat cushions, depending on how collapsed your driver's side one is. Add a small stiffener like a 4"x6" (or bigger) chunk of thin plastic, or cheap berber carpet between the foam and springs to keep the seat springs from cutting into the foam. Free butt comfort goes a long way on the longer trips. If necessary, you can buy new seat foam from genosgarage.com. (bookmark that site--they're awesome)

Those are the main things off the top of my head. You may also want to just spend a Saturday afternoon under the hood and crawling under the truck and look for exposed hoses and wiring that you could put some split-loom on for some abrasion protection. It never hurts and may keep you from being stuck on the side of the road some day.

Good luck! Thank you very much for your service, and enjoy retirement!

Mike