how to predict water and propane useage is pretty much a guess. With DW&I, two grandkids, daughter and SIL we have gone 6-8 days with a 35 gallon black and 70 gallon grey tanks, with no problem. Including quick showers.
In contrast one time on a 6 day trip when I still had close to 30% of the black tank and grey tanks left with the above "passengers", the couple we were with with 1 kid had filled the 40 gallon black, 60 gallon grey by the end of day 4 and had to hook up the trailer to dump the tanks and come back and park again! On day 6 when we left I still had room in the tanks.