Tom,
I've used that very one.... not great. Having an older van we always have a couple of dehumidifiers at all times, one on the shelf above the door, one in the bathroom. The best we have found are called '151 interior dehumidifier', i normally buy them on ebay for a couple of pounds (normally buy 10 at a go), when you first put them in you would think it isnt working and then after about a week it is amazing.
Although we bought an electrical dehumidifier and we plug it in whilst in the driveway for a couple of days each month.
Dee.