A kit? It's not that easy. However, go to the RV store or book store or Amazon or...? and get a copy of a book called
Managing 12 Volts (How to Upgrade, Operate and Troubleshoot 12 Volt Electrical Systems) by Harold Barre. After you've read the first half of the book, you'll feel like you don't need a kit and that you can do this. ....And, you can!
If you only want a 120 AC system, read Caseydog's simple system here:
viewtopic.php?f=30&t=37732There is one kit that you can get for trailer running lights, though, and it is available at most any auto parts store; or, a set usually comes with your trailer chassis kit.