How Do I Register TM30 and Obtain a Residence Certificate Without a Rental Contract in Thailand?

I've a question about TM30 and Residence Certificate... my inmigration office is Chiang Mai in case this is important.

Probably I will leave my current place and move to my gf's place in a couple of months. My gf lives in a rented house with her daughter, rental contract on her name, etc.

As I'm not going to pay any rent, I will NOT be in the rental contract.

So, how to register TM30? Have the owner of the house go to inmigration to register me as a "guest"?

And, moreover, when I will need a Residence Certificate, how to obtain it without a rental contract?

I'm sure many people is a similar situation: foreigners living in their partner's or partner's family's house without any contract. I assume there can be a letter or something like that stating the foreigner living in the house.

But, how to to when the house is rented for the person who invites me? Should the "letter" be submitted by the owner, by the tenant, or by both?

I said "probably I'll move..." because if this could be tricky (thai owner off the house is living in the south, that's the reason he's renting his house in CM) I will stay at my current place - with my TM30, my contract and my really kind landlord living 5 minutes away :D

TIA