At 400-500,000 miles you may want to consider going ahead and changing the Tensioner and both belts if you haven't already. It's a common failure point and the belt beaking while in use can tear coolant hoses off or break the fan. Also, and maybe more experienced mechanics can chime in on this one, consider changing the transmission fluid and rear differential fluid. If you had a cummins isx I'd recommend IMAPs, delta P, cleaning the doser, etc but I dont know much about the sensors on a dd15.
If you haven’t done so. Belts and Tensioner and Idler pulley Def filter, clean sensors Coolant flush, I know it says 600k but guaranteed that coolant is trash if you run a coolant sample thru a lab. 3 box service, trans and Diffs If you have the DT12 trans it was due at 300k. They say 500k on the oil in manual transmission and tear ends. That oil is worn out and contaminated by around 400k. But these long drain intervals look good to end users and gets components to end of warranty before failure. Think planned obsolescence. Change fluids early and often. power steering fluid and filter. simple and cheap. Yearly item to change Have your CAC (charge air cooler) checked for leaks. Have you been changing your air drier cartridge yearly, like your suppose to?. If not, why not?. It’s cheap and easy and prevents many problems.
Get your emissions services. New sensors, new doser, new DPF filter, and get your EGR cooler tested for leaks regularly. You can just change the DPF filters these days. Costs around $2100 at the Freightliner by my house.
I don't want to put words in his mouth but I think he is trying to tell you is that you bought a brand new truck and you did well. You are approaching the point where you're going to have to begin replacing items. This is the reason why fleets get rid of their trucks between 300 and 500000 Miles because that's the point where you begin to have to replace parts and it starts getting more expensive and time-consuming. They get rid of their trucks at that point because they're still low enough mileage someone will buy them but they know if they keep them it's going to start costing them money, not just in repairs but down time. What parts you ask. It's difficult to tell because every truck is different but here's a few things off the top of my head that are possibilities of what you may need beyond that type of mileage and into the future. Slack adjusters, air cans, s cams or calipers, kingpins, springs, airbags, steering boxes, tie rods, spring bushings, torque arms, cab and engine mounts, starter, alternator, hoses, belts, sensors, air switches, wiper motor, heater motor, air dryer, weatherstripping, door locks, window channel, air brake valves, radiator, thermostats, leveling valves, air lines, accessory drive,, fan clutch, air conditioner, charge air cooler, exhaust, wheel bearings, air compressor, wheel or engine or differential seals, water pump, oil cooler,, wiring issues, fuel injection issues, transmission issues, ECM, injectors, head gaskets, Turbo, one box, ... and on and on. To the point being made is that you've done very little to your truck because you had a brand new truck. Now you either buy another brand new truck and keep working with little problems or much more than the list that I've written here is what's in your future. When and exactly what your problems will be is unknown. What is known is that positively without a shadow of a doubt you will be spending money to replace items in the future versus buying a new truck and and rolling Non-Stop trouble-free, or at least that's the way it should be.