We live in Benicia and have the same climate as the OP. In the wet season, he will most likely have problems if he tries to move it, obviously. But if he just leaves it on the dirt/grass, the moisture will shorten the life of the tires.
The original owner of our house poured drive strips about 30' up from the sidewalk to the fence/gate to the side yard. The strips stop there, which is kind of odd, but is fine as that part is about 3' higher than the street. We placed only wood chips in the parking area behind the gate at first. This worked fine for the TT we originally had. However when we bought our current MH, even a little bit of rain caused a major problem. I almost ended up calling a tow truck when it got stuck. When we returned home from that trip, I pulled the chips aside and placed 3 - 4" of gravel in the ruts and then covered it back up. Where the tires and jacks always end up, I left the gravel exposed and made the "pads" level with each other. This will hopefully keep the tires a little drier than sitting right in the wood chips or even the ground.