The big thing is to make sure you get a Deep Cycle battery; not an automotive battery. The next thing is to make sure you get the right "group number" that fits your battery box. Typically Group 24 or the larger Group 27. I like to look at the reserve amp minutes and get the best (biggest) number I can get. Bought mine at NAPA auto parts. Everyone has their favorite brand. Just get the right size with the highest reserve minutes you can get.
Also, I recommend keeping it in tip-top conditions using a Battery Tender while your trailer is in storeage.
They work VERY well for making your battery last and keeping it ready to go at all times.