The rule of thumb is ‘the shorter the better’.
There is nothing to be gained by having the exhaust reach to the wall of the van or beyond. The purpose of the exhaust is release the CO gases as efficiently as possible and that means minimal exhaust length (which translates to less stress on the small motor).
The gas is dispersed into the outside air and does not ‘leak’ back into the van. It simply diffuses rapidly and is virtually unnoticeable.
The fear of exhaust gases filtering back into the van is an odd one given that when the heater is being used all windows will most likely be closed.
That said if you have any anxiety around this then we can install a CO alarm for you too.