Originally Posted by neal
thanks for the clarification, I will check those items.
First, you need to ensure that the 12 volt line through your 7 pin connector is feeding power to the trailer. If you are here just hooked up with the engine running there might not be sufficient current flow to charge the battery but you should be able to test 12 volts at the trailer when the truck is hooked up and running.
The point of turning on your truck headlights is to put a demand on the truck battery so the truck alternator steps up the current and voltage available for your towed vehicle.
If a new battery has gone dead in your trailer there is probably something wrong with the electrical circuit in your trailer unless it that has taken place over several weeks.
While you're on the open road with the engine it working RPM, at best the feed to the trailer will be a trickle charger. It could take 10 or 12 hours of Road time to make a meaningful difference in the battery charge level
Best tool is a multimeter that is also a AC DC clamp meter. I need to be able to measure the current flow in or out of the battery. If the battery is disconnected, fully charged and allowed to rest for about 12 hours a good battery will read approximately 12.7 volts. Totally disconnected, a week later I would expect a good battery still read 12.6 volts more or less. The same should be true with a battery hooked up to your trailer if you have a true battery cutoff switch. The same problem keeps showing up time and time again of people under estimating the static draw of their trailer either because they don't have a cut off switch or because the cutoff switch doesn't cut everythingoff.
Short answer a good battery should hold its charge if not connected to a load. Enroute the tow vehicle is barely adding a charge to even a good battery so if you start out without a fully charged battery you're going to arrive in Camp without a fully charged battery. Possibly the biggest mistake universally made is that one battery is enough for anything. One battery should be enough to meet your needs from home until you get to the hookups in Camp. If you plan to get dry camping time out of one battery you are used will have to be very very tightly managed.
Finally if you have drawn the battery down to zero chances are very good that the battery is irrevocably damaged. I would wrap it with a big voltage charge and leave it on the charger overnight then tested disconnected voltage 12 hours later and again 3 days after that. It is still sitting around 12.6 or 12.7 you might have some useful life still left in it. Good luck