Congrats on the new snake!
I'm sorry you are put in such a situation.
But I am glad you were able to save this 'girl' from an obviously horrible situation.
It does indeed sound like she does have RI.
Can you describe your temperatures?
Do you have a thermostat to regulate the heating pad?
A thermometer to read the cage temps?
If she has RI, bump up the hotspot temps to 95 and her ambient temps to 80. Keep her as warm as possible.
I'm sorry to say, but your visit to the vet might have been somewhat of a waste.
Oral medication isn't very effective in reptiles since they have such a slow metabolism. The best way is to directly inject it into the muscle or under the skin. So I would consult your vet or find a new one. Because he prescribed oral, I sense he isn't very knowledgeable in reptiles.
Also what sort of antibiotics were prescribed?
Most of the time a sample culture is needed in order to target what type of infection your snake has. This is the best way to find which meds work the best for that type of infection since there are many types of RIs.