The resin WILL last longer the SLOWER the flow of water through it. So, if you are filling a static tank in the shed through a DI vessel at anything other than a trickle then yes, having the vessel/s on the van AFTER the pump will be more economical for you. For real economy, you need two vessels- you use one to pre-filter a new one instead of just throwing it away when it creeps up in TDS.
Basically, fill van tank with tap water & purify "on-demand" through vessels after pump if you are DI only.