If you go with metal tank, stick with manufacturer
We have a 1997 P42. We are going to replace the holding tank this winter because the hose connections have rusted down to where almost nothing is left for the tank discharge hose to attach to.I ordered a replacement tank from Ezell Industries, Inc., the company that made the original tank. The aluminum tanks are made thicker now, .090 then and .120 now, so it should last longer, although 8 years isn't too bad. Tank cost $299 plus shipping of $33. Phone number is 850-578-2558 and ask for Linda Wiles. Ezell is located in Fla. There three versions of a holding tank made for the P42. Linda will fax you a diagram and you need to compare it to your tank to confirm size and shape.