6 Volt Batteries and Solar Charging

As mentioned, no controller would be needed with that small of a solar panel. The batteries would hold the voltage down and you couldn't push enough current to be an issue. The problem, depending on battery, temperature and a few other variables the self discharge on 2 deep cycle batteries could be as high as 1 percent. That could be a couple amp hours a day on two golf cart batteries. You'll be unlikely to get 2ah a day out of that panel on many days, so you'll very likely be not keeping up with the slow self discharge of your batteries when your rig is parked. If you don't have a shut off switch to turn everything off on your trailer they will run down a lot faster.