Could you take seperate containers? One pure one tap water?
What system do you have? If you have a van setup, fill the tank with tap water, get a back pack and a few barrels and wash down with pure after.
I do alot of caravans mate and they are hard work, unless theres a few of you (i know you said 4) then you wont get one done in 40 mins. I did 6 in one day on my own once, i was shattered after and im young.
If you did have a team id get them to do it like this, just my opinion.
1st man do gutters
2nd pre soak van in wash and wax, i use a weed sprayer and spray them before i wash them, makes it that little bit easier...and looks good to the customer lol
3rd wash the van with tap water and wfp
4th wash down with pure from backpack/trolley
Then move on like that, swapping over day to day. 320 vans...your gonna be there a looooong time, not to put you off, just letting you know what your getting yourself in for

Id say on a good rate youd get 15 maybe 20 done a day. You may aswel write a month off any other work.