I summise you are leaving the Ninja in the van and filling up from a seperate tank. For this you need a Flojet 12 volt demand pump, this has a microswitch which switches on when there is a pressure drop. So the pump won't run when your tank is full, once your inlet valve opens again the pump will run.
You also have to consider your cleaning solution. Putting it in the Ninja tank means it is being constantly diluted as the tank is being refilled whilst you are working. Your options are to use pre sprays only and rinse with plain water or fill the supply tank with solution as well. Formula 90 liquid is a good one as it is suitable for most carpets and fabrics and gives a good result.
The idea of using a windscreen washer pump is a good idea but it won't have the capacity to keep the Ninja tank adequately filled. Hope this helps.