Hi Steve
Caravans have a variaty of different surfaces Flat,aluminium sheet,GRP and acrylic capped ABS a lot of them are finished off with side trims,
For thoses vans finished with these trims and are poorly maintained, your water can seep behind, and not only start to cause damp problems, but can delaminate the aluminium type surfaces, if this happens you are looking at a huge insurance claim,
so make sure you have an insurance to cover this type of work. make sure the caravans your working on and are poorly maintained, are under 5 years old, the trims are fitted onto a non setting type mastic and according to manufacturers this should be changed every five years, if it isnt and it breaks down then water penitration can happen.
There sttutco type surface is like a bubbly surface and can be difficult to get a really good clean, that combined with trims can hold the water and continue to run long after you have gone (Hense your dirty run marks) if you see the guy's replies to first cleaning for windows their comments are rinse, rinse, rinse, you will need an awfull lot of water to rinse a static caravan, I would Suggest you do all your washing with Ordinary tap water until you are sure all the dirt from behind the trims,around windows and doors, has gone,and then give your final rinse with pure water.
Hope this helps, if you need any more help then just email me.