Water pump will only pump when tank is 1/3 or more full.

I'm going to assume that everything is OK. The pump is.

You would be surprised how fast one can go through water. A 40gal tank that may only have 35 gal useable at best can be empty in a couple nights easy.

I had a 115gal tank in my boat. With 4 onboard it would last 4 nights. That is 6+ gal of water per day per person being very careful.