I'm going to replace the pigtail on my propane tank. I know some folks like to use a type of goop, some use the white Teflon "plumbers tape," while others use a specific type of Teflon tape made spec...
Most RV pigtail to regulator joints that I am aware of have an inverted flare fitting not pipe threads. I do know pigtails can be purchased with NPT so I assume that is what OP has.
Most regulators come with a brass adapter as shown here: