it is possible to make a van mounted system with an RO onboard and fill the tank directly (we have done a few) it is recommended to use the larger (and more expensive) 40" membranes, and this way your tank will be full every morning. - these membranes are also extremely robust, giving out (usually) better quality water, meaning the extra cost is soaked up by the fact that you don't use as much Di resin (as i said, usually, as with everything there is the odd strange one)
it also means if you have a huge job to do, you can plug in and be refilling the tank as you work, making your tank effectively infinate in size.
Cut a caravan accessory point into the side of the van (BE CAREFULL !!) and this way the van can remain locked up. When doing this, we usually fit an outlet into the access point as well, allowing you to lock the van up when out on the job as well - very handy.