Not able to access last 13 gallons of Fresh Water tank ?

[Fishin2dMax]

I have a 2018 HC model 305 RL and do a lot of Boondocking. I've notice the water pump is not able to access approx 1/3 of the freshwater water in the tank. The tank will show 1/3 full, yet the pump starts to starve for water. I drained the tank when it begins to starve and every time there is approx 13 gallons of water still remaining in the tank (not from the water heater)? Given the Freshwater tank is only 54 gallons + 12 gallons in water heater (66 total), I'm only able to access 41 gallons; which doesn't last very long.

Before I attempt to drop the underbelly and look for a solution, I'm curious if others had this same issue, and what was done to fix it so more of the water in the tank can be drawn by the pump? Is the water connection to the freshwater tank at the bottom of the tank, or at the top with a draw line extending down into the tank? Your help is appreciated in trying to solve this.