Very intense, penny pinching roommate has partner over 5 nights a week

I live with my best friend, and two others. One male one female. The intense roommate is the female let’s call her Julia. Julia has a partner Gavin that comes over 5 nights a week. He lives in a van. We all very much like Gavin and have absolutely no issue with him being at the house this much. The house is big, he’s very sweet and I like his presence. The issue is Julia and Gavin fight, like a lot and it’s usually instigated by her and it is sometimes loud. She is also a huge penny pincher and will get upset if people leave things on like lights and dislikes that I put a nice air filter in the living room. She requested it not be on since “we don’t need it” but also refuses to let me air out the home by opening windows because it can make the house cold. She said the thermostat must stay a brisk 64 degrees (it is winter in Oregon) and so the house is cold almost all the time. Most of these things I just leave but with Gavin over 5 nights a week, doing laundry here and showering, I find it only fair for him to begin paying utilities. I personally think he should be on the lease but if they breakup I imagine that will be messy. Again I like both Julia and Gavin they r wonderful in public but hearing the arguments in private is a lot. He has now begun getting mail here. Housemate meeting is this Sunday (it’s Wednesday). What do yall suggest I do to protect myself/ also ensure everything is fair financially. I also want to suggest he pay for toilet paper/ do the bathrooms every week since homie takes massive poops and I clean the bathroom.

UPDATE*****❤️❤️❤️

Spoke to her about the fighting and basically she said she understands but made it seem like her boyfriend is now just going to be a house issue? We agreed he has to come over less and if there’s another loud verbal argument he is banned from the home