It could be several things. As tnchuck100 said, how's the water level?
Also, did you buy this trailer new? With ours, the dealer installed a really cheap and small battery that weighed almost nothing. If that was the case for you, then three years out of that battery might be all you get.
Have you run the battery down a lot? Battery life shortens considerably if you frequently discharge it below about 12 volts. Note - it will still light your lights and such at a lower voltage but not good for the batteries life to go that low. Do you boondock often?
You should really trickle charge the battery while in storage and off from the trailer. Or at least check the voltage on a monthly basis to see if it is losing charge. The longer a battery sits with a low charge, the worse it is for the life of the battery.
Your current battery may or may not be in good enough shape to be re-charged.
If you need to buy a new one and if you boondock often, then I'd buy a bigger true deep cycle battery. There are threads on this forum that discuss amp hours, etc in much more detail.