Semi truck washes will get you started at least with a good cleaning so you can then go to the next step of wax etc. I use Blue Beacon, they charge $20 - $25 for just the trailer $35 - $40 for both truck and trailer. I don't know why it varies but the price fluctuates. They will try to sell you additives and I have never purchased any of those (various rainx, and protectants).
2012 Epic Voltage 3905
2003 High Output Dodge Cummins
As some have said maybe look for a big rig wash and detail shop. I just did mine and 350 is a bargain. I did a show quality detail and spent a good 20 25 hours. I am picky though. I even put Rain-X on the windows.