If your van uses the ALKO IRS Suspension (Independent Rubber Suspension), leaving it sit there for months is doing it damage.

Thr IRS system uses an axle tube fixed to the chassis of the van – it doesn't move like traditional axles. The trailing arms attached to each end of the chassis tube are the parts that move. Inside the tube is a long piece of fluted rubber which provides the suspension and dampening. 

Leaving your van in a stationary position for extended periods causes the rubber to sit in one position – with all the weight bearing on one side of the internal rubber. Over time this can cause the rubber to become out of shape and become detrimental to its performance.

For longevity, if you're leaving your van stationary for any longer than 4 weeks, it is recommended that you jack your van up, such that the weight in not bearing on the wheels.