We have a 30 gallon grey tank and the sensors will say it's 2/3 full pretty quickly... But it isn't. Our family of 4 can go 3- 4 days dumping our dish water in the grey tank and some shower usage before it's full.. with just me and my two kids we made it 5 nights... But we also are active and weren't at the site the whole time. And we honestly didn't use the shower a lot since we were swimming every day in nice clear rivers.. by the way the grey tank said it was full on the 2nd day. It lies
Some things I do you help it last:
Navy showers when you take one. If you have someone in the family unwilling to do that send them to the shower house (my DD is one).
Low flow shower head... Got a nice one from Amazon that both has good pressure and reduces usage.
We wash dishes outside with a couple bins I've had since tent camping days. One gets soapy water and one rinse. I use as little water as I can get away with to clean the dishes. Dump the water down the sink when done, but it really isn't much water at that point.