Uniseal grommets, including the 3" and 1 1/2" sizes, are designed to create a watertight seal without the need for additional sealant. The flexible rubber material compresses tightly around both the pipe and tank wall, forming a secure, leak-proof connection. Using sealant is generally unnecessary and can interfere with the grommet’s sealing ability by preventing proper gripping. You should ensure the tank holes are correctly sized, smooth, and clean before installation for best results. Regarding the 3" and 1 1/2" barbed ABS fittings, once inserted into the grommets, they are but not permanently locked in place. You can typically rotate the fittings slightly to align them precisely with your existing RV ABS plumbing lines. This flexibility makes it advisable to wait to fully connect the barbed fittings until the tank is positioned so you can achieve the best alignment. Using rubber couplers like Fernco connectors afterward will complete the connection securely.