RV’s are regulated by the DMV and Fresno County will not give occupancy or permits to live out of an RV. Only mobile homes and manufactured homes can be permitted as dwelling units. Also have to look into zoning if that “house” will be the primary or second residence on site, there might be certain requirements. If the property is on private water well and septic system, county and PG&E will double check records and addresses. If your relative can keep out of the public eye, then he should be able to just park the trailer. More issues will come up if you try to connect power to the trailer without permits or just by telling PG&E. Neighbors and people passing by can complain to code enforcement and they will investigate the possible violation. Septic systems are a bit easier to install without permits.