Fresh water tank not totally draining

[Sagecoachdriver]

I have added a hatch to visually see my tank level with markings. I drained it to add fresh water for our trip tomorrow. For some unknown reason, the water level dropped to only my first marking which is 3.5 gallons. 3 days ago when I drained the tank it was empty visually. Could the drain be blocked by a piece plastic hole cutout for the filler, air vent or drain fittings?
I understand that all the water can't be drained out, but how do you folks that winterize deal with residual water left in the FW tank?

[Bama Rambler]

Is the hatch on the drain side of the tank or the opposite side? If it's on the opposite side, the vehicle could be slightly tilted away from the drain side.

If it's not that, it could be a piece of debris partially blocking the drain.

[Blackhat6mike]

My freshwater tank was full of plastic giblets that kept stopping up different faucets and valves and the freshwater pickup bung was 2' off the bottom of the tank. I couldn't lift water when the gauges said 1/3 full, just sucked air. I blocked the pickup bung on the side, installed a "T" on the low point drain and rerouted the freshwater pickup off the T. I can now drain it dry either way, but still get pieces of plastic trash now and then.

[MNtraveler]

Stick a pipe cleaner or piece of clean wire up through the drain and see if you can dislodge anything?