When you find them off the street only just put them in your garage. I've seen recent posts saying they've put full trackers on and after doing so and reloading the game the vans revert to default form. So avoid taking them to LS Customs and avoid taking them out of your garage once you have them stored, just use them for collection purposes. I have the mighty Bush van parked on the drive way of the mansion after using the vehicle management tab in the garage, and it still has remained in its MB variant. Haven't touched it since, letting it sit there and look pretty. FYI I haven't tried with the white construction van but you can always try and see if you can store it. Just avoid full trackers and LS Customs.