The first thing to make sure is that your truck's charge line is working. I'm assuming the 7 way was on your truck when you bought it which means you have the factory towing package. If you look at the diagram below, you see the pin on the 7 way plug that provides 12 volts DC to charge your trailer battery.
The problem you might have is Nissan doesn't always include the relays need for the charge line to work. If you look at the picture below (which is for an 04-05), you'll see 'relay 2". This is the relay that provides 12 volt power through the charge line to the 7 way plug. If your truck is a different year, you can look in the owner’s manual to see where the relay is supposed to be. If it's missing, you can get one from your Nissan dealer.
Once this is working, you'll need to rewire the trailer with a 7 way plug to run all the exterior lights and charge the battery. When that's complete your Nissan will charge the trailer battery whenever you turn your ignition switch on.
That is a basic 12 volt charging system. Many people will simply use the tow vehicle to charge the battery and nothing else. However, if you have access to 120 volts AC you could add a converter (takes 120 volts AC and converts it to 12 volts DC) to charge the battery whenever you plug into 120 or you could use a portable battery charger like the ones you use to charge your car battery when it's low.
A converter installation looks like this:
This is a very general explanation so if you need to know anything more specific let me know.