Replacing black tank knife valve Holiday Rambler Vacationer

I had a leak in mine, which has the two tank pipes, each with valve joining together. No flexibility at all and the screw on valves have rims that need a quarter inch of space to come out. I bought new valves bit postponed installing because I will have to cut a section of pipe out and put in a rubber joint. Meanwhile, someone here suggested putting a little vegetable oil in each tank - would you believe, that stopped the leak!