I've got a 22 foot van as well. It was drinking juice at first but then I realised this was because I was trying to force the acceleration that I would get while not towing. Forget that idea, you've got a good ton tided to your butt. I demonstrated this last weekend when I towed the van down to a site about 25 miles away and back again and the Hippo has used no more fuel than it would have done without the van on. Generally, take it steady, keep the accelleration down, keep the speed at around 50 but no more than 60, use the terrain to your advantage and when on a hill assent let the speed drop back so you don't have to hoof it. That way you'll get the result you want. Just remember the more you push that right pedal the more juice you burn. Simple as that.
The hippo is a great caravan tower. The four wheel drive really helps on the grass sites but as Davie said, take care of your clutch. The lack of a low ratio gearbox is a let down when shunting the van about. Having said that, check your van to be has automatic revearse brake release. Mine has and it makes things so much easier.