We have the same problem every winter when we head south - the trailer and tow vehicle is covered in salt and road grime from our southbound trip and I can't stand leaving it in that condition!
One year, once we arrived in warmer climes, we took both truck and trailer through a truck wash (mainly cleaning 18 wheelers.) It did a good job and we were in and out in ten minutes, but id did cost something over $60 as I recall. That included - supposedly some sort of wax rinse.
But usually I just wash it at the first opportunity when we are in a campground. Of course some have rules against it but many permit it - some only the trailer, not the tow vehicle, I guess because you can quite easily take that to a carwash.
Others have no rules against washing trailers, and some say you can wash with a bucket and final rinse with a hose but not to leave a hose running. I supple among other things it might depend on water shortage issues, water pressure issues, and the possibility of messing up your neighbours campground.
We have stopped at some campgrounds that recognize the desire/need to wash RV's and provide a separate wash area.
We have also stopped at some places that do permit it but want you to check first with the office and may charge a small fee.
I generally try to abide by the rules, but will admit that - especially if we are only making an overnight stop, I might surreptitiously flaunt the rules after dark, planning on pleading ignorance if I get caught -something to do with it being better to ask for forgiveness than asking for permission! I do hate to be seen pulling a filthy AS! At other times I might just give it a miss and hope that our next night's top location has more liberal rules!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.