Often overlooked but much easier to build in at the beginning than to retrofit. Apart from just having some fresh air the ventilation system will take into consideration any condensation issues (shower or in a cold / humid environment),removing heat / etc.
Vents to the van are always potential sources of dust, smoke or bugs finding a way into the van. Essentially the less opportunities for dust to enter the better, as well as other hazards. If you have a gas cook-top / fridge, or hot water system then by law there will be a permanent vent allowing the fumes and heat out. This is why vans and caravans have a vent in the door as well as one behind the fridge and hot water system. When planning if you can place two or more devices close together and create a single exit point then that is beneficial. Alternative locations of appliances can also help, for instance placing vent toward the front of the van, or at a higher position then this also can help reduce possible dust ingress.
To almost eliminate dust ingress points you can have more electric appliances and do away with gas altogether. If this is done at the planning stage the only vents you may need will be in the shower area, and behind the fridge.