[JRTJH]

I'm a firm believer in the "belt AND suspenders" insurance when it comes to black tank fittings....

I have a "terminal valve" and a cap with the 3/4 inch garden hose fitting....

That way, even if the "terminal valve leaks, there's only 1" of space beyond the valve and a small cap to remove, not a 3" gully washer amount of outlet to deal with ...

As for "3 or 4 year old trailers with virgin black tanks".... That's a "almost guaranteed way to have dried out valve seals"..... Once they're dried and the valve blade is moved over the brittle rubber seals, it's almost certain that you'll be replacing the seals, not from use, but from dry, brittle rubber being mechanically torn by the valve blade as it pulls its way over the brittle rubber.....

Bottom line: It's very likely that using the toilet prolongs its life and "keeping it virgin" makes for early failure...