When we received our 2021 Caravel
16RB and before we used anything, I used a 5 gallon bucket to fill our 28 gallon combo black/gray tank and monitored what the meter on the galley wall showed. I put in 28 gallons of water and the meter showed 100%. I then continued to fill slowly with an additional 3 gallons until water appeared at the flapper of the toilet. No water at the shower drain (which I expected to see). Our meter and black/gray tank are accurate.
On the other hand, our 23 gallon fresh water tank only holds 21 gallons. I confirmed this by filling it full to overflowing, then drained into 5 gallon buckets. After a complete draining, lowing the tongue to get as much out as possible, I refilled with a flow meter and got the same result (20.9). AS owes me a larger FW tank.
Finally, we have stretched these capacities while dry camping to 10 days, we are currently on day 6, and tanks are: Black 63%, FW 40%… and FYI, 2 battleborn batteries at 12.8v, spotting sun through trees (180w on roof, portables almost useless), 1 day of clouds and rain. Going home today and getting some sun on panels too. We make it work for us, very comfortably.