Treat her as you would a baby puppy
Take her out on a regular schedule that gives her opportunities to eliminate a little more frequently than you think she needs - always go with her, praise her wildly for pottying outdoors. And DON'T give her the sort of freedom that will allow her to have accidents indoors. It may seem to be stating the obvious, but your problem is exactly this:
Originally Posted by maynard001
The accidents only occur when she sneaks off to a quiet part of the house when we're home.
A dog that isn't fully housetrained (and she patently is not) should never have the freedom to sneak off anywhere out of your sight. If she does, then she'll pee - end of story. So do as you would with a baby pup - keep her right within your sight at all times
. Freedom to roam is for housebroken dogs only. When you simply cannot watch her (you've got to go to the bathroom yourself sometime), then use your crate.
If your house isn't well designed for restricting her to the exact same room as you at all times, you might consider tethering her to you (works well with adults). She cannot sneak off anywhere if physically connected to you
Keep treating her as a baby for housetraining purposes (schedule, praise, always in your sight) and she'll soon catch on. Far quicker than most babies do too.