I was under the impression that all tanks of certain gallons had to have the water fill valves assembled above or near the tank for filling. It is no different than the units for doing pre treats and slab treatments for structural pest controllers. The state requires this filling apparatus.
If you are caught without one on the tank...............this is a violation.....in the State of Arkansas!!!
I made one out of 1 inch PVC with a ball valve at the connection. The fill spout is affixted at least 18" above the opening of the tank. horseshoe clamps attach the pvc to the trailer floor. The hose connector isn't hard to find at lowe's or home depot.!