The Cummins ISB engines don't require a coolant filter. They don't have the counter-bores/sleeves/liners that the piston travels up and down in as the large block diesels do, so the coolant additive from a filter isn't needed nor do you need to check coolant for the additive. The purpose of the additive is to prevent cavitation of the liners which, if allowed to happen for any period of time will cause a leak through the lower seal and the fix is very costly.
Having said this though it is important that you keep your coolant system full and clean and flushed periodically to assure top performance.
As a side note you should also keep an eye on your CAC and radiator stack for accumulated oil/dirt build up from the engine blow-by as it will cause you and over heating condition.
Hope this has helped and have a Happy Thanksgiving.
Tom and Barb
2000 Bounder 39Z