Fresh water tank.

I'm new here and not sure if my problem belongs here.
I just returned from a trip where I had to use my fresh water tank in my Evergreen 5th wheel. I had to fill before moving to my campsite and topped the tank until it was over flowing.
We ran out of water in 2 and a half days. Normal usage has always been 4 days. Took the 5th wheel and filled again, topping off as before. Both the filler and air vent had water coming out.
We used very little water but everything acted as if we were empty.
I thought the pump was bad. I bought a new one, installed it. No change.
The sensors on the tank were showing 1/3 both times.
After the first fill they showed full. After the 2nd fill they showed 2/3rds.
I got home opened my tank drain and a large amount of water came out. I would guess some where between a 3rd to a half a tank.
Does anyone have any ideas?
Could it be the pickup line?
I cannot access the tank from above.
It will be a huge project to drop this tank and I physically can't do it, so I.E expensive repair.
Any help or ideas would be appreciated.
WWBELL