as for me and my water in the truck. I run a 60 gallon bladder and a drill pump. Pull up to the spring, and let the spring fill a 5gal bucket,then use the drill pump from the bucket into the bladder in the bed of the truck. It works well and allows me to have plenty of water for the weekend trip when truck camping. We also have a camper and this setup makes it east to refill the fresh tank when we don't want to move the camper.
For only 15 gallons, I would get a bladder and not worry about anything fancy. But the bladder in a rubber tote to help with truck organization. Throw a drill and drill pump in the tote if you believe there will be a need to fill up during your trip. Easy and cheap but works well.