If you have an extra 12volt auto battery that would work fine. If you want a new keeper, most use 12 volt
27D deep cycle marine battery. Looks just like auto battery. There are thousands of post about batteries and types.
If you are hooked up to the trailer the battery won't be doing anything. Unless the trailer gets separated from the tow veh. Then the breakaway switch (you do have a break away switch) uses the trailer battery to work the brakes and stop the trailer. Never seen it happen but sounds exciting. Try not to think about it..... So for now any well charged 12volt auto battery will get you home. If you are lucky the tow veh will keep it charged along the way. When I park at night I always unplug from the tow.