Folks - I would bet the OP asked his question probably because he has no way of washing the trailer himself. He needs to come back and clarify.
I am in a similar predicament. My trailer is kept in a storage lot where, because of price, moving is probably not an option. I live in a house with a driveway yes, but the house is on a small cul de sac which means I can't bring the trailer home without having it in the way on the road, and it is too long for the driveway which is steeply sloped anyway. When I had my previous RV which was 19' long total I used to bring that home to work on it on the weekends, and it got washed regularly.
So maybe the OP is actually looking for answers on how to get it washed other than at his place of abode.
OP - I did a quick Google search of big rig washes in the Riverside area and did not come up with anything suitable in the 2 minutes invested. I suggest you phone some of the truck depots (Loves, Flying J, etc.) in your area and ask them. Truck wash places don't always have an Internet presence. I used one closest to my trailer storage lot once on the previous small RV and they did a great job for $30 or so. The 2 guys used brushes and a soft fan spray on the power washer nozzles. I called the same place and asked them how much they'd charge for a 30' trailer and the guy said $50. Faced with no choice I will be doing that probably twice a year.