I have a couple of NOCO battery tenders too.
Interestingly enough, mine won't charge a dead battery. I left the door ajar on our Chevy Tracker for 3 days, which killed our fairly new Crappy Tire battery. I hooked up my battery tender and left it on for 24 hours, thinking it would bring the battery back to where I could start the car.
Didn't happen.
I ended up using my little emergency battery pack booster. The Tracker fired right up and I took the car for a long drive on the highway to charge the battery.
When I got back home I hooked the battery tender back up (battery showed 25%) and the next day the battery was back to fully charged.
That's "no spark" tech. A lot of the new "smart" chargers, like BT and Noco will not put power to the clamps until they detect that a battery is correctly hooked up. This is a safety feature to prevent the clamps from sparking and causing fires or explosions (hydrogen gas from batteries is explosive!)
Each brand/model of charger has a different minimum voltage threshold - in this case your tracker battery was discharged below the minimum voltage (usually around 0.5-1V) so the charger didn't detect it, and therefore wouldn't charge. What you did is the correct solution; attach another battery (or booster) to boost the voltage a bit, then hook up the charger.
Solstice; it sound like your situation is a bit different. The charger was on and charging, but a camper battery is typically pretty big, and a NOCO G1100 is only 1.1A (or close). It would take days, maybe close to a week for a charger that small to fully charge a depleted camper battery back up to full.
For example; when I brought the Camaro SS home, the battery in it was totally flat, I hooked it up to my Ctek 3300 (3.3A) and it took about 2 days for it to come back to a full charge.
Something with an output as low as the G1100 is best on powersport batteries (smaller), or as a maintainer on larger batteries (cars, campers, boats, etc).
