Does the battery charge when you are towing, or not? Because of the long distance from engine bay of towing vehicle to trailer mounted battery you may be getting a voltage drop sufficient that the battery will not charge. You can overome this by fitting a dc to dc charging module, close to the trailer battery, which will step the voltage supplied to the battery up to about 13.5v This should ensure your battery is being charged every time tyou tow the trailer.
If the problem is that you don't tow thye trailer very often then you may need to put it on acharger from time to time. Overnight about once a month should suffice.