The reason that you are seeing continuity in all the pins on the trailer plug is that the signal is going through the bulb to the ground and back through the other lights to the pins. Almost no resistance through an incandescent bulb when not lit up. When power is put to a bulb it heats up and that is when you will get resistance.
Explaining electrical to a novice is not easy . Great job 👋👋maybe explain led bulbs to conventional and use trailer battery as source for plug confirmation on camper (isolate) vehicle from trailer.
We have several multimeters that we recommend (and have) on our Amazon storefront: www.amazon.com/shop/myrvworksinc The Fluke 325 is often Darren's go to for electrical troubleshooting. And of course our test lead set! myrvworksstore.square.site/s/shop
It’s much better to back probe connectors than to “tap in” and puncture the wiring insulation which will cause possible corrosion issues later…..But we’ve all done it 🫣
