I always squirt some WD40 into my trailer connector on the truck side. Wiggle it around some then lock in place... This just cleans the contacts - a good non-conducting grease is probably better but all of us RV'er will have a yank of wire, a roll of duct tape, and of course a can of WD-40...
To troubleshoot picking up one of these 7-way test plugs really is neat as this lets you know right away whether the trouble is on the truck or trailer side with the running lights... I got mine from AMAZON but I'm sure they can can be found at an auto parts store. ETRAILER.com would be another source...
Roy Ken