Exhaust gas in coolant?

EGR cooler is place to look first. When DPF is full or engine brake event exhaust gases pressru is high and can pressurize cooling system. When exhaust gases pressure drops some coolant can go inside EGR cooler then in exhaust or in engine if EGR valve open. Coolant evaporates water in hot DPF and coolant chemicals plug DPF even more.
Disconnect EGR valve from EGR cooler and look for coolant traces.

 

I don't trust those kits. I probably still have one in the stuff from my old repair shop. Signs of an exhaust gas leak into coolant: Cooling system will be pressurized within a couple minutes of starting the engine. Once warmed up, with the radiator cap off, you will see constant bubbles coming through the coolant, to the outlet (they may not come out till the thermostat opens, because the thermostat can hold the "air" behind it).

 

Re reading your original post, you have zero symptoms of an exhaust leak into coolant. Even if it was leaking once the engine hot really hot, you would still have low coolant in the system, as the "air" would push it out. Because it is displaced by "extra" air, it won't pull back into the system from a non pressurized reservoir (which only works based on standard expansion and contraction of coolant with temperature.

You're looking for a solution without having a problem. Drive the truck.