All outside, you can wash in the cold, although you have to learn when and what you can do. Where I am you can't wash first thing in the morning, but give the sun a few hours on a truck/trailer surface with hot water you should be fine into the 20 ish `F range. Wash with out sun or heat, it will freeze and cause you issues.
I can't get the same volume on fleet washing that I can do in the summer due to the temps and lack of daylight. I get more work outside of fleet washing though because of the cold temps and dirty roads. An example I have a heavy haul customer that uses 3-4 large goldhofer trailers regularly in the winter. Every time they come back with a trailer I get a call, so about once a week I am out there. Trailer takes a day to wash depending on the salt/sand and amount of frozen snow I have to deal with. These are charged hourly of course, but in the summer I don't hear from them because the roads don't cover them with all the sand and salt.
I do say no if it the job and the temperature are not a good or safe (Ice build up makes surfaces slippery). I am not afraid of the cold though, layer up well and make sure you take breaks. I have worked down to -40`f before, not exactly a day I wanted to remember, but the job was 2 months of 6 days a week 12 hours a day job. I would never make an employee work in that, but I guess as the owner and an operator I am on a different level of crazy.