Some people swear by this stuff, but there was no way I could ever justify the cost, especially having watched them applying it out of a garden sprayer.
A good wash (most use CT18 truck wash or similar) followed up by a quality polish every now and again is a cheaper and just as effective way of caring for the exterior of your van.
As regards to the interior, you can by scotchguard in spray cans and apply it yourself for fabric treatment.
Trevor.