Not sure if you actually need water proof vs. water resistant. I've used small 12 volt pumps on weed sprayers (mounted on ATV's for years). We put them through some pretty tough conditions (think rain, dust, fire). Also, had them mounted on the back of fire trucks to refill foam tanks. Can't say they never failed, but I can only recall replacing one...
TwoXentrix has a good point about winterizing the system if you put it outside/under the van.
As an example: https://www.sprayersupplies.com/8000...CABEgKkqfD_BwE