If your charger requires sensing 12 volts to start charging, then either use jumper cables from your car/truck, or back the truck to the trailer, plug in the 7 pin light harness. This will give a limited amount of 12 volt power to the battery, and allow the charger to start charging, even if it needed the 12 volts to start charging.
You should be able to know by now. If you go out to the trailer 4 hours after it was plugged in, and the lights now work, you are OK, no need to do anything else. The charge should be reading full by now (12 hours after your first post). If not reading full, try to jump start the battery, or connect a portable charger to the battery.
Your built in charger should be 45 amps - give or take about 10 amps, so is much more powerful than the average 12 volt battery charger.
Your battery will discharge quickly. The CO and propane detectors draw about 30 amp hours daily, and can discharge a pair of batteries in about 7 days.
Also check your battery water level. If you left it charging for about 30 days, it is time to check it. If fully discharged and then recharged, time to check it now!
Good luck!
Fred.