Utility trailer lights

[Tollypop]

2017 Sunseeker 2400sd.
Turn either flasher on and all rear lights on rv flash. No lights working

on trailer. With running lights on on the rv, no lights on the trailer.
Trailer disconnected, four pin connector checks out correctly with Fluke meter. Power and ground checks out correctly between four pin connector and lamp sockets on trailer. Help! I'm stumped!

[RamblerGuy]

Sometimes circuits check okay with a multimeter but don't work. Bad connections can pass the tiny amount of current from a multimeter but can't pass normal current. The most likely culprit since all are affected is a ground. Test by running a wire from a trailer light to the tow vehicle battery ground post. If the lights work with a good ground, leave it connected to the tow vehicle battery and move the other end until you find the bad ground.