I have a food truck with two small sinks and an 'on-demand' water heater. Our water tank is mounted on the roof of a 10 foot high step van. The water tank holds 30 gallons. Is it possible to just have the tank work by gravity or do we need a water pump. If by gravity, how do we prime it so it flows, or if it doesn't could it be because the water is not an absolutely straight drop. Please give me the best advice you have. I've tried everything I know but obviously common sense must not work in plumbing or I'm just totally clueless. Thanks in advance.
Susan

You fill it with a hose? If so, all you need is gravity to run the water out. Use a 1/2 IP to 1/2 Faucet connector and you are good to go. There must be a tank vent, possibly open or an AAV.

Brian is right. U need a vent to let air in as water flows out. Try leaving fill cap off and see if this helps. If this works add a pipe with elbow at top to keep dirt out. Attach screen on elbow.

Chuck