Need help in how to get a rescue to bond with our family

We rescued a mixed breed (part Great Pyrenees) and she’s about 5 years old. She was emaciated when we got her and weighted only 50 lbs. She’s putting on weight and is eating but prefers to be in a back bedroom by herself most of the day. She doesn’t mess in the house and goes outside and then comes right back in. We are almost finished fencing in the yard so she can’t get out. I think she had been found outside in a field in north Florida and that probably accounts for her being terrified of thunder. She just shakes so bad and you can see the fear in her eyes. She let’s me brush her but I’m not sure how to make her a part of the family. I hate to see her by herself and not sure if I should just let her stay by herself until she’s comfortable with us. We’ve only had her about 2 1/2 weeks and she has come around some but I have no background on her so I’m letting her go at her pace but want to make sure that’s the right thing.