Problems with Girlfriends Dad wat to do.?

Ok so here is the situation. Me and my girlfriend live with her dad where we rent a room we pay him rent each month and buy our own food. but no matter what we do he always moans at us about the bills being to high when there not and always makes us feel unwelcome when his girlfriend is over because she wants to move in but wont whilst we live there. he always eats our food and uses are stuff. I have only recently got a new job and cant afford to move out yet until i pay a few bills of ecetra. The house is also partly owned buy my girlfriends mum who has her own place because she left my girlfriends dad a few years ago. I would say my girlfriends dad is fairly well of but he always tries to take advantage of us and treats us with no respect. The only glimmer of hope I have is that my girlfriends mum owns some of the house as well and is happy for us to live here. but my girlfriends dad seems to treat everything as his when it isnt. Im just a liitle confused on how to deal with the situation?