I use this all the time Jasin. I buy the gallon jug and usually a spray bottle of it or 2. I haven't however used it on a really dirty trailer. I always prewash the trailer then wash/wax it again with the Aero stuff then dry it off. Don't have to do all that but I do. Where we stay here we've got some guys that use it that I have wash/wax my trailer every time I'm here. They do carry their own water supply.
If your trailer is 1/2 clean I would think you could use it (maybe 1/2 gal/2 spray bottles) and take care of it. My concern with a dirty trailer has always been trying to clean it and having fine grit/dirt/sand eat at the finish while I was "dry" washing. So far using it has really kept the outside of the trailer looking nice, shiny and slick. I really like it.
You might check where you stay about washing down the trailer. Most places don't allow it, including this one, but if you talk to them and explain what you are doing I've had them give the go ahead. This place said to pay them $5 and make sure I told anyone that asked I had to pay for the water.