A couple questions here. and info. My tanks do not drain -via drain valve- completely and I cannot suck out all fresh water from tank- pick up line is not at bottom. as I have water running out of drain when pump quits working. If I dry camped more I would investigate and drop pick up line farther down. But with this unit I get full hook ups 90 percent of time
1)- did it take a long time to fill tank at home? maybe only got a few gallons in it -reason ask- Alot of people have had problems with fill lines twisted or too long and loop down, vent line plugged and won't let a lot of water into tank until you blow out vent.
2)-drain lines for tanks are located up from the bottom of tank.-- so you may still be some water in it. Mine when filling will take water for 2-3 mins before it starts to run out drain. When I travel with drain open and park on a side ways slint, more water will drain from tank. Regardless of which side I park on. I always run some water into tank with drain open and let a bunch of water drain before I close uop and finish filling.
3)- is your valve open next to pump going into the fresh water tank. may still be in winterizing setup. --or you may have an airlock and not able to suck up water.
4)- tank sensor are not acurate, I don't know why they even use them. in all tanks in trailer are never right. My fresh water tank reads 1/2 when I am right full and water pouring out of tank. I checked mine out by adding water via 5 gallon jugs and found out the tank held almost as much as they said. Guages are wrong on all my tanks.
5)- example, grey water tank reads 1/2 and water is backed up into bath tub.
Just a few idea and after this happened, I would definatly check tank monitor, or open drain valve briefly to see if water run, before leaving home