Unauthorized RV on property

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Hello, we have a moderately complicated situation and in the process of seeking legal counsel.  I would love your advice as well. We are in CA, a tenant friendly state. For personal reasons, family crisis, etc, my husband rented a property in a city 2 hours away from where we are to the first family that was available to take it. To make it easy (I wasn't there at the time), he did not charge a deposit.  He also knew they had credit score issues.   Forward ten years later, he did a walk through a month ago.  The tenants moved their dad into an RV parked on the side of the house.  According to the neighbor, the dad has been there at least 2 years.  This was done without our consent. They have not missed a rent payment, but every 2-3 months the payment will be late with back and forth communication on when they'll be able to pay.  We received a letter from the city threatening fines if the situation is not fixed related to the RV.  This is for 'person may be occupying a recreational vehicle on our property and this is violation of municipal code (someone living in an RV for more than 72 hrs).  One idea is to continue 'to be nice' and try to work it out, or send a letter with formal tone about the situation.  We have also considered selling the house but I think that's just trying to run away from the problem.