Keep ontop of the drainage, put a brush or similar over to stop the crap going down the drain but allows water though, I always give them a quick blast with the lance once or twice during the clean to keep on top of them and to make sure they're flowing ok. This way if you do find they block up you'll know it was already blocked before you got there, I always carry drain hoses and they work a treat if any drains block.
I have also got a myudsucker which comes out as a last resort, but this is very rare..... I have thought about wet vacs in the past, they are a great idea but.... then I thought, how much gear do you really want to carry around with you? I personally like to be very neat and tidy / organised, there fore don't want to be tripping over lots of gear in the van.
Another angle, more fore commercial users is to use a flat surface cleaner with a mudsucker attached, like the karcher frv 30's and falch skaters, these are great devices as there is no generator / vacuum needed, just an extra hose pointing towards a drain, however a great deal of flow is required as the cleaning nozzles will need a minimum of 15lpm, (21 lpm in my opinion for the perfect clean), plus an extra 5 to 10 lpm to work the venturi on the discharge port