I had been worried about my fresh water being empty even though the sensor showed it half full because my water pump would not work. The mechanic showed me with a flash light inside one of my basement storage areas, that you could see it was really half full. Turns out my water pump takes a long time to prime and you have to open the taps to help it if you have not used it in several days.
However, do NOT leave your water pump turned on as you drive or park or all of your fresh water might drain into your grey water tank if you have left a tap open, as I learned recently. (Still learning, as you can see.)
I have 30 gallon grey and black water tanks and can take a brief shower and do a handful of dishes every day for four days without dumping grey water. Black water lasts me at least a week and I use plenty of water but hardly any TP. (Learned from that mistake way back in October if you want to see my panicy posts at 11:00 pm one night when i had a TP clog and could not use my toilet!)