We live in Oro Valley and have stayed at Totonaka in SC for many years. Great place and Joseph, the owner, goes out of his way to make his guests happy. If you need anything just ask. It's like having a concierge at a hotel.
It's an easy 5 hour ride from Nogales. Stay on MX15 all the way and just watch for the signs, especially going through Hermosillo. Get your visas and permit for the trailer at the Immigration and Customs (Aduana)stop, 21km into Mexico. You should take enough pesos for the tolls going south. They no longer take dollars at he booths. You can use your ATM card to get additional pesos in San CArlos
We also travel with 2 dogs. You need to get a health certificate for them from their vet. We always just carried their shot records and had never been asked for a health certificate until this year. No problem entering Mexico but the US officers asked for the certificates coming back into the US.
Fill your vehicle with fuel before crossing the border. Fuel is now more expensive in Mexico.
San Carlos is a nice, easy introduction to Mexico. Relax, have fun and enjoy