All Cummins engines designed for over the road have the filter heads built right on them. The spin on filters come with DCA additive or without. For a gasoline engine engine you would want a filter without DCA. We use the filters without because we use extended life coolant. We have a recycler that I bought when CF was auctioned off for a song. It probably cost a fortune new. You could build something like what you are describing, but why not just put a remote filter in line with your heater. That way you are constantly filtering your coolant and never have to drain it. Our recyler is basically a glorified holding tank to drain coolant into when we have to work on something. It beats the old fashioned way of draining it into a wash tub.
