The brass pressure relief valve is electrically isolated from the aluminum tank similar to the dielectric unions used with a household water heater, You would have direct contact from brass to aluminum with the brass petcock.I had the same idea as you after loosing one plastic plug and breaking another but the local RV shop recommended staying with the plastic plug . I've read several articles on this subject , some say NO problem, other state it lead to leaks
and premature water heater failure.
I guess it depends on who you want to believe
I know from personal experience as an electrician , the effects of copper and steel coming in contact . The results were not good