R.V.
Since she goes outside while off leash you can teach her a command for potty. When you let her outside to go give her the comand go potty, or whatever you want to use. While she is going keep repeating the command. when she is done, reward her. Eventually she will learn that this command means you had better go now. Do this for awhile, a week or so, try taking her out on a leash and giving her the command, when she goes you make a HUGE deal out of it. LOTS of praise, treats, petting, everything. It will probably only take one time and she will get it.
This command works very well if you travel or visit places where you need her to go at a specific time or place.
Good Luck!