Hi Steven
I used a pressure washer a couple of times on our 'van, but to be honest, as you have to work with reduced pressure there was no real advantage. I still had to work over it by hand to get rid of all the black streaks and winter gunge from all the nooks and crannies. So I don't bother with it any more. There are too many areas to be aware of as well. The roof lights don't seal, so water can 'bounce' under them and get the interior wet. You have to be careful of all the vents, such as the fridge or heater vents or again you'll end up filling the inside of the 'van with water. As you've said the windows seals are another area to avoid. And any mastic that's not tightly sealed or is getting a bit hardened with age can easily be blasted out and so will cause all sorts of problems, so avoid all joints.
By the time you've avoided all the above areas there's not a lot left to use it on. Plus the spray back produced on a caravan wall, especially on a windy day, can leave you as wet as the 'van.
Nah, it's easier with a bucket and sponge and just use a hose with a fine spray to rinse it off afterwards. Car shampoo for the 'van and Cif cream for the marks.