"...I know she wouldn't keep a decent change of clothes in the backpack..."

We keep a spare change of clothes under the back seat of our truck. Undies, cheap "workout" suit, a couple days worth of personal meds, and some hideout money. All sealed up in a vacuum bag. You might try that with her spare clothes; the bag sucks down small enough to not take up much space, and is just enough of a pain to open that she might not dig into it when she runs out of clean socks or something. And don't tell her about any money you put inside...