The method Joseph describes will work and is certaintly the simplest way to make this connection, however it does carry a bit of risk. If you forget to disconnect the tow vehicle's battery when at your destination, you will be drawing current from both the trailer's on-board battery and the tow vehicle's battery. This could potentially leave you with a two dead batteries if not careful. To avoid having to remember to break the connection every time, you can use add a constant use solenoid to the tow vehicle's trailer charge wire circuit. The charge wire connects to the large lugs on the solenoid, the smaller lug connects to any circuit that is hot when the ignition is on. This way, the charge circiut is open any time the key off preventing accidential drain of the tow vehicle's battery.
Here is a photo of a constant use solenoid. Note that it looks like a regular starter solenoid, but it is different. Starter solenoids are designed to be energized for only short durations. Constant use solenoids can carry the current constantly, hence the name.