There are a few things to address here as far as husbandry that once done will likely bring her appetite back around.
Reptiles must be at the proper heat for them to desire to eat. For RTB's that's around 80 degrees. From the information above, it sounds like it's about 10 degrees too cool. Heating pads do help to heat snakes, as long as they're laying on them, but they must also be regulated. The heating pad should be around 90 degrees and keep in mind that that does not raise the ambient temperature. If she's not laying on the pad continually, she won't receive the heat that she needs. Additionally, the humidity needs to be around 70%. Some terrariums need to be partially covered with a solid sheet of wood, glass or other insulation that allows the humidity to stay that high. There are super cheap thermometer/hygrometer combos that you can place in the middle of her cage to give you an idea on temp and humidity. Pet stores carry them, as well as stores like Walmart and Home Depot. The humidity can suffer a little bit for a short while, but ambient heat absolutely must be where it should be in order for her to feel like doing anything she's supposed to do and that's likely why she's acting lethargic.
As well, I'd also like to address her food sourcing. At 3' in length, she's not getting enough food for her size. You can certainly maintain feeding mice, if you prefer, but I would recommend a small to medium adult rat once a week. If you stick with mice, increase the frequency that you're feeding. Offering her 4-6 mice at a time may cause regurgitation. She has a lot of body size to make up for and this, also, may be contributing to her lack of appetite. When their calories intake is low enough, the body can enter a type of hibernation-like state where the snakes will conserve energy.
Once you bring her ambient/hotspot/humidity percentage back to where they need to be, give her about a week and then offer her prey. Do be cautious about feeding her live, especially in this state, as it's not uncommon for rodents to attack predators when they're not interested in eating. In fact, this is why so many herpers have gone away from feeding live: they have the capacity to injure. If she refuses mice and rats once at a higher temperature, consider using a hamster or gerbil. They are a much different item and often the switch gives snakes the added desire to feed. Right now, focus on getting her to eat vs. feeding larger quantities. If the temp remains low and she feeds, she may also regurgitate.
I've attached this sheet below to give you more husbandry info:
Another thing I would consider looking into is a CHE (ceramic heat emitter) instead of using UTH (under the tank heating). these are more dependable for keeping snakes warm and where we want their humidity, plus it makes a hot spot possible (heating pads don't when it comes to ambient heat).
Keep doing as you're doing with allowing her to stay in her bath. Move it closer to the heating pad to keep the water warm, if that's where she prefers to stay. Keep an eye out for any wheezing, sniffling and bubbles coming from her nostrils. There's a risk of respiratory compromise at this time too.
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