You need more info, soap wash vs brushing is a big difference in time and money. Also how much mud if there is any are on the trucks. I used to do dump trucks every 2 weeks in which the mud would take 45 mins to remove before I could get to making it look pretty. Very little in costs as water is not expensive, so soap and gas being the only costs, but the time was long.
You have to find out how clean they want these, and how dirty they get before I would set a price. Also keep in mind winter months have different cleaning requirements if the trailers/sleepers go into snow. I know you don't have to worry about snow, just trying to give you as much info as I can.
Good luck, maybe set a price for the first cleaning and figure out how long it takes you and go from there. You will get quicker, but factor in what your time/material/time to pay is and go from there.