You should use a "T" designed for the job. Extend-a-Stay used to be made by Marshall Brass, (Also Extend-a-flow, a bit cheaper for units where the tanks can be removed) but I think they dropped that line.
Stay-A-While is a nearly identicly product, I think Mr. Heater makes it
If you go into most any RV store and ask a Person over 40 for "Extend-a-stay" good chance they will get you where you want to go
POL connectors (Prest-O-Lite) Screw INTO the tank valve, and are left hand threaded, hence the nickname Put On Left
Acme has a big Plastic NUT that screws ONTO the tank valve. Other features as well
Most Mounted tanks should have POL if I'm not mistaken, but I may very well be Mistaken. ACME is used for Removable (Trailer) Tanks. NOTE the standard ACME valve will accept a POL fitting.