I owned a 1980 H30 many years ago. The boat did not have a holding tank or bag, so I added a 15 gallon square bag that I bought from West marine. I had to cut one of the inlet/outlet holes but it was not too difficult and never leaked. I installed the bag in the port side V-berth compartment just forward of the bulkhead next to the head. The most difficult part was getting all of the sanitation hoses in place. They don't bend easily. Also, make sure that any fittings are correctly sized for the hoses as different manufacturers publish ID vs OD diameters for the barbed fittings.