I think you have the same problem that I had with a KZ Spree. The pickup hose was on the side of the tank about 1-1/2" up. When the water drops to the top of the port, the pump will then suck air. If the tank is 6" high, then the top of the port is about 1/3 of a tank. To make matters worse, the tank was supported with only 3 straps which allowed sagging, in the unsupported areas. The sags wasted even more capacity. Even worse, the tank was vented straight down which caused extreme water loss while traveling.
Fix for my trailer:
1. Vented tank well above water level.
2. Plugged side supply port.
3. Attached the supply hose to the drain hose on bottom.
4. Added two more straps to reduce sagging.
