Ok, so this maybe long, and may or may not help.
I know and understand how your bf feels. My mother was a narcissist ( maybe check out raised by narcissist sub-reddit). It was just me, little bro and her.
She was by far the biggest contributor to my C-PTSD. She verbally, physically, emotionally and financially abused mainly me (the scapegoat, as didn't want kids and was mentally ill - wonder why?) But also abused to a slightly lesser extent, my favoured younger bro (golden child).
We had no choice but to go to college/university (one of her choice and for a course she approved of), had to get a high paying job with high social standing (lawyer, doctor, veterinarian) so she could say she produced us?. She wanted us to get a house in the country, with lot's of land, with a 'nice little cottage' for her on the land, so she could live next to us, raising her grandchildren, all free for her. Whomever got the good job first, would be the 'lucky' one to take care of her. She wanted to retire at 50. She said it's our 'duty' and 'responsibility', she repeatedly told us this for as long as I can remember.
However, she had cancer and was disabled for 13 years (since I was 11 years old), I lived with and cared for her that entire time, she also abused me the entire time. She died 7 months ago, and all though i have lots of baggage I feel free for the first time in my life, I haven't cried about her death or missed her, if anything I have celebrated. I finally see colours in the world, not shades of grey. But I sure have cried many times over the years and since her death over the 'idea' of a mother, and 'what could have been'. I've been slowly coming to terms over the years that my mother wasn't a 'proper' mother like my friends have.
I loved my mum and couldn't see the abuse and scale of it for a long time. My family and friends told me she was abusive, I honestly think I didn't want to accept my own mum could abuse me. I finally opened my eyes three years ago, when she tried to split me and my bf up, when that didn't work she abused both of us. The last three years of her life were the hardest as I started to call her out on things, started saying no. She hated the loss of control and her abuse not only escalated, she abused my bro more and she stopped caring who saw it (she was careful to hide her abuse before). I only stayed as long as I did because I knew she would be dead very soon. But after everything she did to us including the days up to when she died (threw an object at me and told me to get out of the house and she hated me), I wish we'd have left perminately when she threatened to kick us out and make me and my bf homeless the first time. I'd prefer to sleep on the streets than be abused.
You have already given at least $9,600 to your parents that 'got eaten by the house'. Now they want an extra $7,200 per year on top! Imagine what you could do with that money - car, education, retirement/savings fund, wedding or even just treat you and your gf. They will only stop asking for money when you're bankrupt and homeless and are of no more use to them. That is also when you won't see or hear from them anymore.
You want a family that loves, cares, respects and appreciates you. Sadly, you're parents aren't that. They only want you when they want something from you. If your parents, the house and the debt suddenly vanished, How would you feel? Relieved? Free? Guilty? sure, but that's only because they trained you to feel that way.
You have the power the make it go away. You need to say no. You need to stop giving them anything. You need to leave and stay minimal/no contact (because they will hound, guilt and manipulate you at every opportunity once you stop supporting them). You need to build a new life that focuses on your happiness. You need to build a new family who love, care and respect you for no other reason than being you (I have 'adopted' my best friend as my sister, lol). Your first member to your new family is your gf, who sounds lovely and supportive. You need to take care of yourself, your future and your future family.
Nobody deserves to be treat like this, especially by their so-called parents. Please let us know how things go.